CAT 279C Wont recover tracking speed

Hi all my friend has a CAT279C 2011 model the problem is if your tracking along in second or first gear go up a hill speed would reduce (which is normal) what the problem is you get to the top of the hill and on to the flat is the speed wont resume engine revs go back to where they were but no resuming of speed and when you go down the other side of the hill the speed does no resume to previous( to get around the problem you simply stop and then continue on and the problem goes) for example going 12 kmh on the flat speed drops to 4 kmh going up the hill stays at 4kmh on the flat and stays at 4 kmh down the other side of the hill (no other hydraulics used during the fault apart from steering ) the machine has had a new main drive pump fitted (track driving pump) and has been to CAT Dealer and been checked out at no charge they put 15 hrs on it so they done a lot of testing with no luck thanks to all that view and reply