yes we changed the wirers and plugs and put on anew distribor cap
Ok great. The first thing I'd like to do is check the rotor play. Pull the cap back off and turn the rotor by hand. If it moves more than 1/4" by hand it shows there is wear on the distributor drive gear and it would need to be replaced.
If that's ok the next place to go would be to make sure the distributor is in sync. There is a cam sensor (pickup plate) in the distributor to tell the engine controller where the cam is in rotation, and a crank sensor mounted behind the right cylinder head to tell the controller where the crank is in rotation. Over time between timing chain stretch, cam and distributor drive gear wear, the sync will slowly change. The ignition timing on your truck is electronically controlled by the engine controller. Turning the distributor does not change ignition timing, but getting it in the right spot is very important. If it goes either side of 0 by about 10 degrees it will start running bad and can set this code. Putting the distributor in the right position on these is called "setting sync". This is best done with a dealer scan tool, but can be done with a voltmeter. You are setting the rotation so the cam sensor (inside the distributor) toggles its signal voltage at the perfect time. It needs to be within 10 degrees of 0, if its goes much farther it will set a code for intermittent loss of cam or crank signal (P1391), and if its far enough out it will actually send spark to the wrong pole on the distributor cap, or it will fire to two wires at once. Here is the procedure to set the distributor on your van without using a dealer scan tool.
NOTE: DO NOT ROTATE THE ENGINE COUNTER CLOCKWISE. IF THE ENGINE IS ROTATED BEYOND THE MARK, RETURN TO STEP 6 AND REPEAT THE PROCEDURE.
Once this is done, disconnect the battery to clear the codes, or use a code reader to do it if you have one. See if the problem returns. If it does then you have either a cam or crank sensor signal that is dropping out intermittently. It could be the wiring or either sensor, most likely the crank sensor.