1993 Isuzu Trooper no downshift

The Anti-theft light on my 93 Isuzu trooper LS (DOHC engine) started to flash while at highway speed. When I stopped, the autotrans did not downshift to the lower gears. I can manually put the trans selector to L to get it into lowest gear. Since then, at times when turning off and turning back on ignition, the Anti-theft light does not flash and car will move in lower gears and downshifting happens normally, but at other times the Anti-theft light will start to flash either on start or while driving, with the autotrans then not downshifting properly. Is this a problem with the anti-theft unit, the Electronic Control Module, a sensor associated with the autotrans, or the autotrans itself?