[MFP8055]

I'm experiencing weak forward movement on my 1610 after it has operated for around an hour. Here are the full symptoms:
- When cold, there's a noise coming from the hydro unit when in motion
- After warm-up, everything is fine with lots of drive power
- After an hour, it's difficult to go up hills and there is a slippage or lurching in forward motion. Backing down to neutral and then forward helps, but drive power is still low.
The tractor is obviously a little old, and I doubt the hydro unit has ever been serviced. It has new Dexron fluid, new rear end oil, and a new drive belt.
Is there any way to repair or replace the hydro unit? How about adjustments? I have an owners manual, but it's not a lot of help. Thanks from a new forum member.