Many things could be causing your problem, including a leak caused by a worn or loose line, transmission modulator, or possibly the regulator.....you would need to troubleshoot with the vaccum pump tool to be sure....otherwise it is a guessing game.

I had some rough shifting problems. My vaccum switches are leaking (there are two of them). For now I've plugged the line going to these switches until I can get a pair. It shifts pretty well which makes me wonder if I really need the switches...from what I can tell, at full throttle, one switch opens and pulls the vacuum to 0 to ensure a hard shift for a forced downshift. You definitely don't want a soft shift (causing wear) in this situation.

If you put a vacuum gauge at the line going to the transmission, you should see a variable vacuum reading with throttle position....I think mine varies from about 5 to 12 with the throttle.
(this is with the vacuum switch line plugged).

There is quite a bit of material on this forum you can search and study on the subject.

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