One thing I have found to be a problem is the vacuum port for the transmission/engine controls at the brake booster line. You will often find vacuum at the port, but the vacuum has to also be considered a "volume" not just a vacuum measurement to be measured. A restricted port (by caked on deposits inside the booster line) can allow a good vacuum measurement, but a lousy "volume" of vacuum. You need to locate the proper vacuum nipple (port) on the brake booster line, there is usually 2. It's fairly easy to figure out. One is for the door locks, engine shut-off system, and climate control, the other is for engine control devices, such as the trans modulator in question, also i believe the EGR valve. Find the port supplying vacuum to the engine control systems and after CAREFULLY removing the rubber hose from the port, try to clean out the port, either with a small drill bit (1/16" or less) or a tip cleaner for a torch, or maybe even a straightened paperclip. If this makes a difference (and it may very well require an immediate readjustment of the vacuum modulator) then a proper repair would be to replace the booster line and then do a final readjustment of the modulator.