My 2005 Grand Prix GT is starting to have a problem. when I stop at a light or stop sign it sometimes slips out of gear and then bangs back in when I start to accelerate. I have heard that this transmission has a history of this problem due to a defective seal. Is this true and how costly of a repair is this?

About the only thing you can do, is check the fluid level, and condition of it....

Next thing would be to check for transmission codes in the PCM.....

There are many TSB'* out there on various transmission problems.....a transmission code is a good starting point....