Hi-I usually don't reply to these things but I jut got finished dealing with the same problem as you. I have a 2005 Chevy Impala with just over 75K miles as well.
The problem is most likely the Pressure Control Solenoid, and you will have an error code of 1811 in your computer. I made the mistake of taking mine back to the dealer where I bought it who made me pay $100.00 to tell me that it was a $1400K fix. Apparently this is a problem in this make and model, and the dealership said they probably just had to clearn it.
My advice is to take it to a respected transmition place, and have them look at not only the PCM but also a drum that is connected with it. They may have to replace it or reseal it because the two problems are connected. It will be an expensive fix because they have to take the entire transmission out, so it it a lot of labor. My guy did it for 1000K, which replaced the part and rebuilt and resealed the drum associated with this problem. The dealers will def try to screw you, because they never told me about the related issue, and I'm sure (I may be jaded) they were looking for me to come in in a another few 1000 miles.
I tried changing the transmission fluid too, hoping that would solve the problem, but once that computer registers the error, it will keep happening depending on the temp of the engine, which is why it will start doing it, but go away if you turn the car off and start again. It sounds like yours might be slightly more advanced than mine, though. Also, my check engine light did not come on, and the onstar disagnostics emails did not tell me any issues were happening.
So, my advice would be take it to a transmission guy you can trust, and plan on at least a 1000K fix.
Monday, November 23rd, 2009 AT 4:19 PM