I am stumped

I originally posted a thread that my tranny had 1 and reverse only. Well after the car sat for couple of days a guy came and looked at it and it was shifting fine. He said it may be that the filter needed to be changed or the cooler lines where clogged. Now it shifts fine until the car is run for 10 to 15 mins. then the problem occurs again. Let it sit for 2 to 3 hours and it is fine again.
Any ideas beside the ones the guy had mentioned??:banghead:

try a transmission service on it first, to see if that is why it is going into what is called "limp home mode" the computer is sensing a problem and locking you out of the other gears to protect the transmission. If the check engine light is on scan the system to see if you can determine the cause of the problem.