Turn the vac modulator CW for a sharper shift. That may be all you need. The tranny only needs vac for the downshift; without vac you should get a clunk at each downshift which seems to be what you are experiencing. It bleeds off the vac for the upshift.

So maybe you have the vac modulator turned too far CCW and it is giving you soft, soft shifts (although I do not know whether it would cause the shifts to slip).