What do you think this is? My tranny slips out of overdrive on the highway and will not go back in. Only happens once in a while when it is hot out. When the car sits for a while it works properly once again.
When it acts up and I let off the gas, the rpm's will drop off to idle speed. The motor will free rev until it gets up to driving rpm(around 3000 at 65-70mph with no overdrive) :(

Could be a bad converter lock-up soleniod, or possibly a 3-4 shift solenoid. I had a 93 Bonneville with a similar problem. A Pontiac dealer technician drove the car and said I needed a new transmission, $2,300. I took the car to a GOOD local shop, and they diagnosed a bad shift solenoid, total cost of repair $385. Take the car to a GOOD independent shop.

Thanks for the response mike! One thing I would like to add is when the transmission acts up it gets louder. More of a hydraulic fluid noise.

Unfortunately, if you are getting strange noises, you could definitely have more severe problems.