Sam,
Yes I think you are losing oil in the shuttle clutch pack somewhere, but these "really strange things" keep coming up. One last attempt to determine if you have A37546, A38031, A38300, or A38434 valves. A37546 is the machine pre serial # 8314666 valve. All use the same pressure regulator valve, but only A37546 uses a different flow spool. It actually looks different with only 2 machined lands instead of 3. CNH shows it updating, I believe, to the later spool valve, if you want to order a new one. This is on page 101C.
Look at your flow spool( the one the clutch pedal moves) and see if it has 2 lands(A37551-early) or 3(A38033-late), see the attached picture for the difference in appearance. My manual shows oil flowing differently around these 2 different flow spools, possibly giving either a charged or uncharged converter with the clutch pedal down. Phil