What could cause the transmission of a 2001 Nissan maxima se to rev up after 2nd gear.

Question

I have a 2001 Nissan maxima se, and the problem I'm having trouble with is the transmission. After about 2 hours of driving the transmission starts to rev up after 2nd gear. I would just pull over to the side and let it cool off for about 30 minutes and it drives better. What do you think is the most likely problem. There is no check engine light on.