Trans slip/trans program update...

I took the car
to a very reputable shop recommended by the BMW dealer. He checked the trans and engine computer and only found one code for an o2 sensor. That morning I had unplugged the TCU to see which one I had and it didn't go into trans program on the way because I drove it very carefully. Would the TCU clear the codes after being disconnected for a few minutes? The mechanic said it went into trans program mode when he started it and when it was idling. It has never done this to me and It never did after the visit. He suggested checking the A/M switch. Did that and it was very clean in there. I ordered another one and a shift indicator. hope one of these fixes the problem. I refuse to believe the trans is bad because if it was why does it slip in third but not slip in third when in trans program limp mode. there is no slippage at all when in limp mode. Also, this seems wierd but In limp mode you should only have 3rd gear right? well if I put the gear selector in 2nd or 1st i get 2nd gear, which isn't near as bad as starting and being stuck in 3rd. I can start off in 2nd and then upshift to 3rd or D to get 3rd gear. I am baffled.