93 GP SE running poorly

I just bought a 93 GP SE 3.1 auto. the car runs like it is only running on 4 of the 6 plugs. I took it to a dealership and they said it was the coils. I replaced the 3 colis and it ran good for about 5 miles. The car runs good in the early am when I go to work as long as I dont stop at the store, because if I do then it is back to running on 4 plugs. last night I had to make a couple of trips 1 at 10 pm and again at 2 am, the car ran great in the cool night air. today when I started the car about 2 pm it ran good for about 10 miles and went back to running pooly. A couple of times in the past when driving the car for awhile it will take off and run good for a short time then go back to running bad. Any ideas and help.

How old are the plugs, wires and fuel filter? If they are fairly new, I'd look at the crank position sensor and ignition module, as both can fail when they get hot but work fine when they are cool.

how old is the fuel pump also? My car did the same thing and my pump was goin out.....

When it starts acting up, unplug all 6 wires at the coil packsand have an assistant crank the motor for a few seconds. This is a quick test that tests the integrity of the ignition components. If you see hot blue sparks jump across each of the three coil packs, then the system from there back is ok and you should start looking into maybe fuel injector problem (common on older 3.1's) or plug and wire issue.

Take a close look at your wiring on the motor. I had the same problem and had to replace connectors on the fuel injectors and breaks in the main engine harness. Dont know if it helps but its something else to put on your plate and consider.