Properly conditioned modulator pressure is critical to those old trannies.
There is a rubber diaphram that failed regularly in the modulator. There also is a device called a double lifting solenoid on the top of the trans. There are three positions, idle, full throttle and everywhere else. The everywhere else position is the unpowered state; meaning the solenoid is powered to the other two positions. The solenoid moves a linkage that attaches to the modulator and the three positions creats three wildly different ranges of modulator pressure.
You should verify the modulator pressure with a gauge and if much less than spec verify that the linkage is in the right position. There is an access cover under the tunnel rug on the right side. Makes a hole the size of a grapefruit at the top.
Bosch Master, ASE Master, L1
33 years MB technician