1995 Chevy Truck tuneup question

motor hesitates under low RPM load. Example: If I shift into 3rd at 20 MPH the motor hesiates when i try to accelerate. Once the speed picks up it runs smooth. Example: When driving at 70MPH with the cruize on and I start up a hill the engine hesiitates under the addded load, once I reach the top of the hill I can accelerate without motor heisitation. As long as the RMP'S stay high the acceleration is good, without heitation but when the motor has a load at low RPM it misses.