i've had that exact same problem on 2 mopars and it ended up being 3 different things....
1. bad ecu/coil.
2. bad reluctor/pickup (make sure you use a brass feeler to set gap).
3. low coil voltage (supply at about 4VDC - which was a bad ballast)...

but the most persistent problem i had was the ecu, that one was exactly how you describe...tried 3 orange boxes and all eventually after a few weeks did it (including just dying after 40mins on hot days)...finally went to a chrome mp box, all gone for 5yrs now, and there really is a big difference between the orange and chrome mp boxes...chrome much much better, better idle, better power...and they work!!

hope that helps!