M1151 A1 funky behavior at the traffic light

[Mogman]

Yeah.... I left it there for a while and then repeated the whole procedure from scratch and all I got was the 12 (three times)

The code 12 tells you it is in diag mode, it will flash 12 three times, then if there are any codes it will flash each code three times then it will flash 12 one more time telling you that is all the stored codes, there can of course be more than one stored code, you obviously do not have any stored codes.
If it were me I would drive it with the TCM disconnected and if it did not exhibit any of the symptoms as before I would try swapping out the TCM as that is the most likely suspect.

 

[Mogman]

The TCM controls the converter lockup on the neg. side meaning it supplies +12V to the control solenoid and controls the ground side, it could simply be a bypass capacitor in the TCM going bad causing the neg side to short to ground making the converter try to lock.
This would likely not cause a code as that TCM was one of the first ones ever designed and was a very rudimentary device.
If you wanted to prove it you could monitor the voltage on wire #924A (pin X on the TCM connector) it should stay at or very near the 12V feed voltage until it would be correct for the TCM to lock the converter, which would only be in 3rd or 4th gear.