Have a 2004 2388 Case Combine with 2200 hours on it that I purchased used.  I've owner this machine for 5 years now.  This Combine will lose speed going down the road and when you come to a small incline it loses speed to the point that it is only going 12 mph.  I was told by the Dealer that they have 2 different transmissions in these, one for 30.5x32 tires and one for 20.8x42 tires, which are the tires I have on this machine.  Two years ago the Dealer told me it was the hydro pump was bad so I replaced it and that made no difference in performance.  I have also replaced the inch pedal needle and seat that same year.  This year I pulled out the transmission and replaced the gears from the 35x37 to the 34x38 which the Dealer said was the problem, because it was geared to high.  So as long as I had the transmission out I had the hydro motor on the side of transmission rebuilt.  Got the machine all put back together and still have the same problem.  Has anybody ever experience this problem?  I am out of ideas, so any help would be greatly appreciated.  Forgot to mention that this Combine also has rear assist on it, now wondering if that would cause this issue.  Thank you.