When i first start the truck it revs up. then going down the street it starts off fine and once it gets to 1.5 to 2 rpms it sounds and acts like its stuck in gear it will eventualy go past or drop back down on the rpms and drive fine for a while can anyone please tell me what this might be. I dont know if this has anything to do with it but i just replaced the water pump, fan clutch, and radiator. can anyone tell me what the problem might be?