Transmission Light Question

[OK Hog]

I have a ’97 XJ6 that has a recurring problem with the transmission light and check engine light coming on simultaneously. The problem typically occurs when slowing into an intersection. The engine idles down too slowly and the lights come on. The transmission locks itself into a safe mode (2nd gear) but is still drivable and the engine sounds great. Once the codes are reset, the car runs well for anywhere between 3 months and 1 day before the problem reoccurs.

The transmission computer was thought to be the original problem and has been replaced. The code that comes up now indicates a problem with an oxygen sensor, but one has been replaced and the others have each been tested and are working properly. One unexplained oddity is that the oxygen bank will sometimes record a negative percentage. The spark plugs were replaced. The main computer was pulled from the dash and analyzed only to find that it and all the wires leading out of it are working properly. It has been at 2 different shops who have both told me they don’t know what to try next. That is why I’m here. Any thoughts? Thanks in advance.