Nissan Transmission

About 2 years ago I got my 1991 Nissan Maxima Transmission rebuilt. Since then, my transmission has worked fine until about 2 weeks ago. I started smelling burned transmission fluid then my car started jerking when I acclerated. After I parked it, and tried to put it into drive, it jus reved up then after five minutes it got into gear but slipped. I took it to my mechanic who did a transmission flush. Initially it solved the problem but after a day it came back. About 2 days ago, my speedometer stopped working but my transmission went back to working fine. Im not sure if I should try to mess with the speedometer if the transmission is back working. Any ideas or help would be much appreciated. Thanks