1997 Avalon 1MZ-FE

I bought a used 97 avalon with 100,000 miles on it. When I drive the car for more than 30 minutes at speed over 55 (rmps over 2000) the car starts to kind of miss and then when i pull over the rpm fluctuates up and down and it seems like the car will stall. If I wait for 5 minutes and start up again, the car takes about 2 seconds to start up meaning that it takes usually a bit longer to start up compared to a regular start up. Then the car is fine again for another 30 minutes. It only happens when I drive the car for more than 20-30 minutes at high speed, otherwise short trips here and there no problems at all. I cleaned throttle body and changed idle speed air valve already. I really can't figure out what the problem could be. Has this happened to anyone? The engine is 1MZ-FE which is the same for avalons from 95-99 and all V6 toyota camrys and the lexus ES 300. IF anyone has ever heard to this problem please help out. Thanks so much.