Don,
It sounds like you have a problem with the VCV on the IP or something related to that. If the tranny was really going out, it would flare at full throttle shifts too. I bet you have too MUCH vacuum at light throttle. Try connecting a vacuum gauge with a "tee" in the line that goes to the tranny (after the VCV). Route the hose into the engine compartment (through the window or something) so you can drive the car and watch the vac gauge. It should be near zero at full throttle, at 15 inches at idle (no throttle), and be pretty linear anywhere in between. If not, you need to adjust or replace the VCV.
Note: You didn't specify which of your cars has this problem. The 1985 model has a more complex system which improves transmission shifting compared to older models. If it's this car, it could be one of several other things too.
Here's an EXCELLENT article on transmission adjustments:
http://www.mbz.org/info/articles/trans/service/
Regards,