I would at least get under the car and check the wire from the OD switch to the transmission. It seems oil and heat and time conspire to destroy the insulation on the wire and it shorts out intermittently.

On our 87 760T I had a recurring problem with the button/shaft on the AT shifter popping off..repeated replacement didn't work; I used a soft nail rather than a hardened roll pin (found out later the roll pin IS available, just have to order it) down in the guts of the shifter detent.

Long story short, I replaced the entire shifter assembly (spendy, but O is it nice and tight if you like that sort of thing :>) and when I took the old one out found the OD wire insulation was like paste...seriously deteriorating/gone in places. The wire/connector is available for a few bux at the volvo dealer.