Moved from the CTS-V Performance Mods forum to the 1st gen CTS forum. Hopefully your fellow CTS owners can help you out here; none of the 1st gen CTS-Vs had an automatic, so nobody in that forum is likely to to give you useful responses.
Could be one or more of several things. I would start with the simple stuff...has the ATF ever been changed? If so, was it refilled to the correct level? Low ATF can cause issues. If the ATF was never changed, it could be severely degraded and/or low.