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 and was given the 1870 code then I took it to a transmission shop and got the transmission component slipping code. 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 transmission. There was no metal in the pan just very dirty fluid. 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, just gettin a valve body, or getting an rebuilt tranny. The transmission shop I actually went in to serviced the transmission because it hasnt been done in the 3 years I had the car. The car shifts fine, and I took it on the highway and it seems like when I get between 50 and 70 it may be losing a little power. Everyone that I've spoken with has said that I would know if the transmission was slipping because it's very noticable. I'm wondering what your thoughts are? Could the P1870 code be for something other than a transmission issue? If not what would be your suggestion on getting it fixed? A new valve body? A whole new transmission? TCC?