Tranny Problem

I have a 1995 Buick Riviera Supercharged V6. I bought it needing a motor. The transmission has been shifting wierd since I put a new motor in it, i dont now if it did it when the other people owned it. when it goes to shift from 2nd to 3rd it will go into 3rd and drop back into 2nd then shift back into 3rd then 4th. It's done that since May of 2007, then a problem developed that it would dog down when I would accelerate heavily and it would really dog down when throttle was fully pressed. I then read a thread on the net to disconnect the vacuum line from the transmission modulater. So i did and it was like a new transmission. So i replaced the modulater and 12 hrs later it was back to the shifting wierd but not dogging down. Now after about 2 weeks it is starting to dog down again but not as bad. I tried disconnecting the vacuum line again but it made no difference this time. I've been thinking it's a bad tranny but have been told that if the tranny is going out to check to see if the fluid is burnt. It isnt, It looks just as new as when I changed the fluid and filter back in may of 2007. I dont have much money coming in right now so im trying to see if it could be a sensor or something, so if anybody knows what it possibly could be, I would really appreciate it if u could share that info with me, thanks.