You could have a misadjusted or stuck downshift cable.

Another possibility, and I've experienced this years ago in a 90 744T, is that there are intake air leaks from the rubber hoses in the lines to the intercooler or off the turbo. The leaks cause the engine to lose some of the boost the turbo is making. Therefore you have to open the throttle more to get the same power.... therefore the trans downshift cable is pulled tighter because the accelerator cable is pulled further... and the trans will shift out of OD at some certain threshold in that condition. So it's doing what it's designed to, in response to the inputs it's receiving.

Check out those hoses, check the downshift cable.

Good luck!
--
::: Rob Bareiss, New London CT ::: 92 244 ::: 90 745GL ::: 90 745T ::: 84 242DL ::: 90 745T Parts ::: Used to have : 86 244DL, 87 244DL, 91 244, 88 244GL, 88 744GLE, 82 245T, 86 244DL, 87 244DL, 88 245DL, 89 244DL!