My 95 Pontiac Grnd Prx overheats after driving 30-45minutes.

Okay! I Have a 1995 Pontiac Grand Prix SE (6 cyl.). It has 156,000 miles, yet the motor was replaced with the newer motor having 60,000 miles on it when the car was at 140,000 miles.

When the weather gets warm or hot outside, my temp gage seems to rise. If it is warm outside, I can drive my car for 45 minutes, then the temp. Goes up above 210 degrees. If it is hot outside, the temp seems to rise in shorter amounts of time (25-30 minutes). I have to pull over and shut my car off for 15-30 minutes until the temp is down, well enough to drive safely again.
I got my car checked out. The radiator is fine. No leaks, no clogs. The hose is fine. I do have an exhaust leak, and the mechanic told me not to worry about that. I also have an oil leak, although the mechanic did not tell me where it was coming from. He said that because I have a bad oil leak, the motor will eventually go out, and that I should sell the car. Yet aside from this problem, my car is great!
I drove my car today, and I checked the engine oil before driving. It was full. After 35 minutes of driving on the highway, I got off on my exit, and my temp gage began to rise. Thankfully I was at my destination so that my car could be shut off!
Please help me. If this problem can be fixed, I'd love to keep my car and not sell it!