Transmission slipping?

After fixing BCM issues and lately having a problem with engine idle (which has temporarily gone into remission) now the automatic transmission is slipping in first gear. The problem is worst when the car is cold. This morning the engine was revving in both reverse and drive, but the car was not moving. After a few minutes I could back up slowly with the engine revving around 4,000 RPMs, then putting it in drive it moved forward slowly until I got it up to 20 MPH. Once the car warmed up, I could move okay from a standing start, but the engine was still revving 2,000 RPMs just to get the car to move. I think it may be 75,000 miles since the transmission was serviced. Should I get the transmission fluid changed or pour a product like Lucas Transmission Fix (assuming there is a filler port) or do both if new transmission fluid does not solve the problem? Or should I trade the car quickly before it strands me somewhere?