sounds either like the faulty TPS signal as previously mentioned, or a bad torque converter. if you smelled burning, i'm guessing that'll lead more towards the torque converter side of the spectrum.

take it to a mechanic and tell them exactly what has been going on, how it has been driving, and maybe even take them for a drive to duplicate the problem. there's a very good chance they'll recommend to have the tranny oil changed, filter changed, and have it flushed. i most definately would get all of these done. they'll drain the fluid before doing the flush, and most likely find plenty of particles. there would be a lot of metal particles left in the coolers, reducing their effectiveness. flushing the coolers will allow for them to cool the tranny fluid much better.