Governor or modulator one of the two is probably the culprit. I would replace the modulator first as it is cheaper and easier to change. Verify that the vacuum hose is in good shape at both ends.
The Governor nylon gear could have gone bad after sitting for so long. They get old and brittle and sometimes crack and break off. You can get a new one from the part's store, on line, or from the U-pull-it yard. Just make sure that the gear is in good shape and there's no wear on the shaft. There is a bushing in the TH400 that if it moves, it will wear the shaft 1" to 1 1/2" on so watch for it. If you replace the governor, don't forget the rubber O-ring gasket on the cover. Be prepared for a blast of fluid when you take the cover off. Sometimes there is, and sometimes there isn't. Better to be safe than sorry (don't tast good at all!).
I could have posted this on my first post but getting home from work at midnight, I suffer from brain fade! Sorry.