4.3 gmc is sputtering and backfiring

I have a 92 gmc with a standard transmission that is backfiring and sputtering after it has been running for a while. I have noticed that on longer trips, 30+ miles, the truck backfires when i go to accelerate on the highway. Also, after it has been running a while, I will stop at a light, When I go to accelerate, It backfires and tries to stall but wont. I have to turn the key off and wait for the truck to shut off. Once it shuts off, I will restart it and then it runs normal till the next light.. I have replaced the plugs recently, thinking that it was a misfire problem. All the plugs looked normal. None were dirtier than the rest. It only does this after it has been running for a long distance. If it is cold, it does not have a problem. I have also tried seafoam in the tank and direct into the carb. I am at a loss? the temp gauge never gets hot?

Try coolant temp sensor left front top of engine when your looking at it has a plug screws into center plate or look in Chilton manual. sometimes if this is gone engine doesn't know if engine is hot or cold. this is not the same as the one for the gauge

prob egr valve