Did you do the check of the intake gaskets?

 

check your spark and make sure its hot enough....also i would check the tps (throttle positioning sensor) sometimes those can make weird things happen

 

The overdrive light and the code for the EPC may be related. The EPC solenoid can be changed by just removing the pan. Your problem may lie in the transmission. You may need to have it checked out at a transmission shop.

 

if it is the tranny soleniod its not that hard to do...you just drain the tranny fluid and remover the pan (buy a filter and gasket set when you do this) and the soleniod is held up by to bolts and a couple wires and you drop it and snip the wires re butt connect them to the new one and put it all back together its that simple but the tranny soleniod shouldnt have anything to do with how it runs just how it shifts

 

Can you feel it shutter when reving the engine in neutral, or does it only do it when in gear? If it only does it in gear, you probably have tranny issues (as otherss mentioned above).

 

Have you checked your coils. Mine did the same thing and i had a bad coil.