With those miles on the counter you should check out your vacuum system,
specially the tubes and rubber connections to the tranny.
But in the first place verify the ATF oillevel in it. Than take out every "restrictor" in the vacuum system. These are the small plastic tubes with a restricted opening in it between two rubber hoses or elbows. If you can't see light through it, clean it with one "hair" of a steelwire brush ( handy to clean the orifices of the windscreenwashers to! ).
Anyway, you should do a search on this forum : there is so mutch written
about auto trannies and the vacuum system here.