1990 lesabre ses problem

when you first start the car it runs real rough, so i have to let it idle for a min or it will stall out. but once i let it idle it is fine to drive until i drive it for about 45 min straight then the temp light comes on. i just turn the heat on and it goes off. but what could cause this. i changed the tps but it still runs the same. any suggestions.. thanks

Low coolant level, stuck theromstat, coolant temp sensor & idle air control are just a few suggestions.

Have you been putting in coolant lately?

yep coolant is fine. the light came on a few weeks ago when i gave it heavy gas to pass somebody.

Where did you check the coolant level, at the reservoir or the radiator? You may have air in the system. Basic diagnostics required here.

i add directly to the radiator when i add coolant