Engine Performance problem
1991 Chevy Lumina 6 cyl Front Wheel Drive Automatic 177000 miles

my car starts up well and drives well but as it heats up ..say about 40 minutes or longer of driving i get check engine light..and when i park the car for maybe 30 minutes or so and then restart the car for my trip back home it runs rough. then the next day it starts up okay and its a repeat of the same again. what seems to be the problem?? catalytic converter or some kind of sensor? also is there a way to make the car run smoother as i do my return trip home? would you suggest turning on the heater and venting the heat from the engine so the engine does not heat up as much?? thank you for any suggestions.

can you smell any fumes when its running rough? like rotten eggs? thats a sure sign of a clogged up cat. but the fact you're heating up in the first place has me wondering about your heater hoses and thermostat. could possibly be as simple as a radiator flush then bleeding it correctly. and make sure you use 50/50. dont add water. use concentrated 50/50...