Starting to overheat

I have a 1998 Pontiac Gram Am v6, 101,130 miles. I got an oil change two weeks ago and then a week later I went to get in my car to go somewhere. It started no problem and got to my destination. When I went to leave the car wouldn't start and had to get a jump, then it started right up. The next morning I went to start it for work and it hesitated then started. Everyday since then, it starts fine, but for the past few days, it would start to overheat some when I would stop at a light. It wouldn't go all the way to hot and stays in the normal range, but its a high normal. It would go back down when I would get on the highway only to start to climb a bit when I stop at a light again. I haven't had my radio or ac unit on for fear that it adds to it, but it starts to overheat without anything on.

what could be the problem and what could need fixed? Also, does the high temps here in TX also have anything to do with it? It is cool when I go to work at 5am then anywhere up to the 90s or 100s when I head home. Thanks for the help.