I had a hunch that might be the case. I've run into this twice, both times on Magnums (same powertrain). Let's see if I can explain it so it makes sense....
When you did the t-stat, I'm sure you noticed that it's a strange setup (t-stat in the block). There is a small passage that runs down from the head, and dumps out right on the thermostat. This is what causes the thermostat to open. This is a small passage, restricted down by the headgasket. Generally what happens is junk left over from the casting process accumulates in this passage, blocks it off, and the thermostat won't open properly.
The first time I ran into the cooling system would cycle wildly (up to 250+ before the thermostat would open). The second time it wouldn't open at all.
Both happened at higher mileage than you would expect (one was @14k, the other at 30k).
Run it up to temp, if the thermostats not opening (very little coolant circulation) then I would be pulling the head off. I wish I had some pictures to show you exactly what I'm talking about.. The fix is cleaning the gunk out. I've tried flushing / blowing it out, and never had any luck.
If it was my car, I'd be pushing for Chrysler to fix it, pretty classic example of a manufacturing defect.
Does this sound how the cars acting (overheating / poor circulation through system?).