CTD2001

My dad has a 2002 6 speed HO. This is his third cummins. His current truck has about 68,000 miles on it. For about the last 15,000 miles he has had a problem with the truck. When he is driving the truck up a hill it goes up the hill with out a problem, then when he comes down the other side of the hill he will let his foot off the gas pedel so the truck is remaining at the same speed, when it is time to press the gas pedal in to maintain the speed once the road has flattend out, he gets nothing. No acceleration at all, even if he holds the pedal to the floor. To correct this problem, he has to push the clutch in, and let the truck idle back down to normal idol rpms. After he does this the truck run normal with proper acceleration ect. untill he comes to a similar situation. We changed the fuel filter in his truck thinking that might be the problem, but it was not. Not sure what to do from here, thought you guys might be able to help us out. Thanks