thermostat?
if the thermostat was messed up, you would have problems with your cooling system, ie - too much coolent get through and your getting cooler temperature readings then average, and etc.

have you checked your coil pack in the distributer, an injector can be screwed, many things could be messed up.

i had the same problem a while back, and, it turned out to be the oxygen sensor, which got screwed by major road flooding a days before the symptoms were occuring.