1999 Pontiac Grand Prix P1870 Code

I have a 99 Grand Prix SE 3.1 engine with about 206K miles. About a week ago my SES light came on. Took it to Auto Zone got the generic code which was P1870 so I took it to a tranny shop and they said it meant transmission component slipping. They took the pan out showed me the fluid was really really dirty, smelled a little burned and said I needed to either get a new valve body or a new tranny. There was no metal in the pan just very dirty fluid. The car drives like its brand new still.I spoke with another tranny shop over the phone and he gave a couple suggestions of either putting a cooling fan on because the P1870 code may just be that the tranny or the fluid is overheating or getting a valve body, or getting a rebuilt tranny. The transmission was serviced after the light came on. Driving in the city the car shifts fine when speeding up and slowing down. I recently got back on the highway with the car and once or twice it seemed like the car hesistating a little, almost like it wasnt getting any fuel. Everyone I've spoken with said it would be very noticeable if my transmission was actually slipping. Could the P1870 code be an indication of something other than a transmission issue?