01 SS transmission

I have an 01 SS, which we bought used less than a year ago. At first, everything was fine, then it started shifting at about 50 mph instead of 40-45, and before long, when you'd shift it from park to drive, it wouldn't move. So we started driving it in 3rd, and it was a lot better, but it seems to be worse in the cold and after it's been running a while, it seems to go away. Also, I've had the engine coolant and service lights come on regardless of when it's serviced, the tire pressure light's been on but I figured out how to reset that, and now the airbag light is on. We've had the transmission checked out, but they're not sure what's wrong exactly, and no codes popped up when we had the computer checked. Has anybody experienced this kind of stuff before?

I've heard of the trans problem before from a friend. In his case, it was the clutch engagement spring going bad. After vehicle was at operating temperature everything was fine. No codes ever surfaced on his either.