In the case of my 1991 vehicle my mechanic changed the modulator valve and suddenly nothing worked right. Truck had 120,000 miles on it.
Wife now drives it.
Four years later my 1992 truck was eating tranny fluid though the vacuum system. By then I had found this site and figuring it to be a pretty reasonable repair I asked my 'new' mechanic to replace the modulator valve. When I went to pick up the truck I discovered it would not shift normally, was given to sudden high revs - a sick puppy for sure. After checking with the local high priest of tranny's we discovered that my mechanic had forgotten to reinstall the one inch pin that mounts on the inside of the modulator valve. Remove the pin and inspect.