Buick Help! Stalls after 20 miles or so:)

1997 Buick Century V6 152,000 miles.

This car has ran great since the day a dropped $500 for it 3 years ago.

It continues to run great until it runs for about 20 miles of driving. Then it starts to stall as I am stopping. I can usually start it right up, but as soon as I put it into gear, it will stall again. I usually only have to wait about 5 to 10 minutes and then I can put it into gear and it is fine.

Sounds like it may be overheating, but there are no lights that come on to indicate this. Could it be as simple as overheating, like a bad thermostat? Or radiator problems? Or could it be something more?

Any Ideas on the problem(s)?