2003 Hyundai Elantra automatic 4 cylinder w/air conditioning.. Car drives fine on short trips around town. After about an hour of steady driving at 50-60 mph, it loses all power, although motor continues to run. Car does not accelerate, almost as if the transmission is disengaged from the motor. No warning lights, no overheating, no unusual sounds.
After turning motor off and waiting 5 minutes, car drives fine again for a about an hour and then repeats the process.