Jeep Grand Cherokee

My son has a 1996 Jeep Grand Cherokee with 176000 miles, automatic transmission. Recently, he has started experiencing problems after the vihicle has been running for approx 1 hour. He notices that it begins to jump and jerk, and he says it feels like you are riding on a flat tire. He never notices this on short trips. He is away at college and has this problem anytime he comes home. We have had the transmission checked and had to have the crank sensor replaced about 3 months ago, for another problem. Any suggestion as to what could be causing this would be appreciated.