Hello all, I have a 1999 E350 mini bus that I'm working in with the 7.3L that has been converted to a 4x4 bus with a used but 4R100 transmission that has a remanufactured sticker on it from sometime ago. It spends its winters on mat-tracks in driving on snow roads at a camp in the mountains. The complaint when it was given to me was that the transmission was slipping and they believed the clutch packs were going out. According to them it started at high gear but got to the point where it was slipping from idle. The camp had dropped the pan but didn't see much in the line of excess clutch material. They didn't seal up the pan and ended up leaking all the fluid out by the time they delivered it to me. I sealed the pan properly and filled the trans with fluid and didn't seem to notice slippage. I drove it for nearly 100 miles with no issues with the exception of one case. I was driving out of a substantial gully on the highway at about 60 and when it downshifted to go back up the other hill the engine started revving and it slowed to about 30-35 and stayed there for the rest of the hill without being able to accelerate any. Once I got to the top I put it in park, checked the fluid to make sure it was still high, put it back in drive and drove another 10 miles back to my place without any issue. I hooked a pressure gauge up since I thought it might be a failing pump but got what appeared to be appropriate readings. Anyone have an idea of where to go from here? I have access to a 4wd Dyno we could hook it to and load the bus up with the equivalent of 4,000lbs to try to simulate using the tracks or whatever, but I'm not sure any other options to determine the cause of the problem.

Thanks for any help

PSI Readings:
Idle:
P - 60
R - 75
N - 65
D - 65
2 - 60
1 - 95

Stall - (Foot all the way on brake and revving the engine)
1 - 140
2 - 145
D - 150