Hi and welcome to JustAnswer!. This is Ed.
Your problem is likely related to a loss of ground to the instrument cluster through a cracked ground solder joint on the cluster's printed circuit board.
Vans with factory vehicle theft systems typically wind up on our site as start-die problems from this sort of issue. If the van was built without factory theft, your only indication of something wrong is that the dash goes flat at times, but with theft, you probably won't be able to go for a ride. We can fix that, you and I.
The most likely cause and fix (I'd estimate 90% or better) is a cracked solder joint in that same instrument cluster printed circuit board I mentioned earlier. More than just a PC board, it's an actual processor, supporting cluster functions AND the inter-vehicle communication network known as the CCD bus. If this network goes flat, the information trying to be received from other sources is interrupted and you're faced with this sort of problem.
The solution is to pull the cluster out and either replace/ initialize a new board (about $800 at the dealer/ me) or fix the bad solder joint yourself (cost next to nothing). I'll send you info on how to do it yourself if you're the adventurous sort. It does involve a fair amount of dash work to get the cluster out and on the bench, but if you take your time and have an aptitude for electronic soldering, this will save you a ton-o-dough.
Here we go...
What you'll need to do the job would be a #2 Phillips screwdriver (I like a shortie and a long, around 18") and then possibly a #20 Torx bit for the screws on the back side of the unit. Most use Phillips fasteners but some were built with the Torx screws, so be prepared for that possibility. A headlamp or good droplight helps, too, unless you're working outside in bright daylight.
And then there's the soldering iron. It just needs to be something with a pencil tip type of end for working on the board, so the options are pretty varied. You can get a soldering "gun" with trigger controls and a pencil tip or just get an always-on soldering "iron" that's used for electronic repairs. A typical electronics soldering iron like that is a lot lighter and handier... as well as smaller which helps when working with delicate repairs like this.
I prefer a low melting point resin-core solder and the smaller the wire size the better so you don't feed too much metal into the solder joint at a time. Connecting terminal joints with a solder blob is a definite hazard in this procedure.
Begin by removing the driver's side lower knee blocker panel, requiring the removal of two lower screws at the bottom of the unit and two at the park brake release handle area. The upper portion of the blocker panel is held in place by clips, so just pull outward.
Your brake release cable will come out with the blocker, so be careful to avoid too much monkey motion because it can become detached at the park brake assembly deeper inside the dash (not good). Leave it hanging or remove the cable at the handle itself.
Now exposed, remove the single screw at the base of the left side vent assembly and pop the vent out, which will expose still more screws. Take 'em out, along with the screws you can now see since the knee blocker came off along the lower portion of the upper dash.
The trim that lives just below the instrument cluster (and above the column) will be freed as those two screws are removed, but clips remain holding it in place. Pull up and rearward and this panel will come out. There will be a switch attached to the left side of the unit if you have traction control which is best disconnected down deeper into the dash (use a screwdriver to release the keeper tab and pull the entire piece of trim to detach).
At this point you should see a screw or two at the lower portions of the dash, so take out anything that's left.. remove all visible screws. Then pull the panel toward you... as there are a number of clips that just need to be shown who's boss. There are no screws from the lower portion of the panel other than the ones you've removed, so it's all trust and pull at this point. Some adhesive or spilled Coke might be sticking the upper portion of the cluster trim to the dash.
As a rule of thumb, if there are no screws showing, your part attachment to the dash is from just friction from clips so go ahead and pull.
Now released, you have the option of just tilting the cluster "bezel" up to the left for access to the cluster or removing the electrical connectors at the headlamp switch side.
Four Phillips screws remain to be removed to get the cluster out, located at the outer four corners of the unit. Once loose, pull it toward you and rotate the top out a bit to get a hand in behind the unit to release the connector. It's done by lifting a tab on the connector and then (pretty much) pulling the whole cluster toward you. It's tight back there and the cluster wiring loom may be tangled with the message center that sits directly above the cluster, so be careful about pulling on the wiring. It could damage the message center (voice of experience talking here).
Now removed, set the cluster on the work bench face down on some cloth (being mindful of the protruding trip reset button) and start taking screws out until the cardboard cover on the back is removed and then the board is loose. This is the view you'll have, courtesy of one of my own JA customers, Dean.
Lift the board carefully to avoid bending it as best you can. Once the screws are removed it's held in place only by friction on the gauge pins, so work it off slowly and evenly.
Now off, rotate the unit toward you with the umbilical for the electronic PRNDL display still attached. It can be disconnected or just left in place like the picture shows. The only downside of disconnecting it is that you have to remember to reconnect it when going back together... =/
Solder up the joints as Dean so aptly demonstrated, then begin the reassembly process. These pins are the ones directly opposite the red connector on the other side of the board that you had to struggle with earlier when disconnecting the cluster from the harness. As the only stressed portion of the board, it figures that these solder joints would be the ones to suffer from years of road vibration and if you have magnification, you might be able to spot a dull ring around the pins in the centers of these joints... the actual cracking that brought you here.
Once the cluster is back together, hold the cluster up and inspect the needles of your gauges for resting on the left side of the faces, as they may have wandered a bit during the operation. If they go back in the van with their needles facing toward the right side of the gauges, key-on will rotate 'em to the wrong way and you'll wind up with a tach or speedometer needle on the bottom side of the stops. Rotating the cluster in the air while watching the needles will get them where they need to go.
Reassembly is just reverse of the disassembly at this point, with no real surprises. Oddly, it seems easier to reconnect the cluster than disconnect it, so going back together goes pretty smoothly.
I'll be here for any follow-up questions or problems that you may have. If that graphic doesn't load, I'll give it another try.