I checked the I&T manual....there are 3 hydraulic filters on the hydro unit, and I think you indicated earlier that you had changed all 3 of them. Here's some additional troubleshooting info from the manual:
Tractor will not move or moves erratically:
Trans oil level low
Control linkage improperly adjusted or damaged
Charge pump damaged
Engine clutch or trans input shaft failure
Neutral valve stuck open
Tractor moves, but lacks power:
High pressure relief valve opening pressure set too low
Internal leakage of high pressure oil
Other symptoms listed are Abnormal noise, Failing to stop in neutral, or Oil overheating. I don't think you mentioned any of those, so I didn't type that stuff in. Let me know if you need it.
Good luck.
Tractor will not move or moves erratically:
Trans oil level low
Control linkage improperly adjusted or damaged
Charge pump damaged
Engine clutch or trans input shaft failure
Neutral valve stuck open
Tractor moves, but lacks power:
High pressure relief valve opening pressure set too low
Internal leakage of high pressure oil
Other symptoms listed are Abnormal noise, Failing to stop in neutral, or Oil overheating. I don't think you mentioned any of those, so I didn't type that stuff in. Let me know if you need it.
Good luck.