transmission slips

We are at wits end. The transmission starts out fine if you put shifter in low. Starting out in drive it goes in and out of "gear" until it shifts to second. We unplugged the electrical plug and it was better but shifted way slow constantly slips so plugged that back in. We did a lot of vacuum line and modulator type replacements but nothing helped there. Tonight I just detached the vacuum line at the intake manifold and the damn thing seems to work fine. I am sure this is probably not good to do so can anyone help here!!
Oh it is a 1994 v6 3.1 and we think it is a 3 speed automatic (shifter has P,R,N,O,D,L on it)

by what your shifter reads,,,,,4 speed trans,,,,,sounds like a clutch pack slipping,,,,you can leave the modulator un-plugged...this controls line pressure,,,disconnected causes trans line pressure to go to max pressure,,, this causes clutch packs to be squeezed tighter with higher pressure ,not a fix but can be driven untill re-build or repair can be done....

another possibility is to replace modulator with after-market adjustable type,,,crank screw in snout clock-wise to almost bottom of range,,4 turns or so,,,,if tranny is not totally shot,,,,this should solve the problem......if doesen't slip at full pressure,,,,this fix has a shot!!!!!!!

Thank you. This has been very helpful. We'll give the adjustable modulator a try.