From the ATSG manual

1. Check regulating pressure, if no regulating pressure is indicated, continue with item 2. If regulator is in order, perform item 3.
2. Clean centrifugal governor and make operable.
3. Disassemble, clean shift valve housing and exchange if required.

When mine did this, it was because of excessive internal leakage from a worn out trans. If the trans has got over 200k on it, it really needs rebuilt.
I have had 3 of these apart so far between 215k and 230k. 1 of them had worn out B3 and K1 clutches, 1 had just the B3 worn out and the other(my 300sd) had all 3 clutches worn out along with a busted planetary gearset.

Where are you in washington. If yours does need rebuilt, I would do it for half of what a shop will charge.

__________________
81 300SD daily driver/project
86 420SEL (sold)
85 380SE(in the graveyard)