Hello,
I was having a similar problem with my 95 2.3L Pickup, 185k miles. I had just replaced the head gasket and after a few months I had the truck stumbling or hesitating. This would occur when I would be shifting from 1st to 2nd on the five speed at first and then later developed into a number of other situations included downshifting while taking an off ramp or a slow climb while on the highway. Sometimes parked on the side of the road because the truck could not continue due of excessive bucking I would rev the engine and it made a sound like it may backfire. It felt as if the gas was being cut off but it would continue to run. I have no tach so I cannot say what the rpms were doing exactly.
It would come and go. Since it did not happen all the time I decided to watch for any patterns that might appear. The nearest thing I could figure is that it only occurred in warmer weather (above freezing). This turned out to be a red herring.
Recently it had gotten so bad that it was nearly un-drivable. I figured at this point that it was bad enough and happening consistently enough so a mechanic might see it in person. I went to an Isuzu dealer because I felt that they may have seen this sort of thing before. After three hours of diagnosis ($250) they tell me that it had been traced back to a bad ground connection on the engine and that the ignition and fuel system were ok. When I looked around the engine compartment I could see that they replaced a few nuts connecting a pair of grounds to the engine. They claimed there are four grounds connected to the engine and I have not had time to see if they changed any hardware on the rest. Apparently when the engine was revved the ground connection was intermittent.
It has only been a few days but it seems to be running a whole lot better. Its running smoother and the problem has not occurred so far. I am cautiously optimistic this was the problem given that when it first started it would occur every couple of months.
It seems the link posted by mpd7 got it right for my truck. I wish I had seen it earlier.