Do you have wiring schematics to look over the prints? My first thought is a bad ground to the up fast coil. I'm not anywhere near a tac 32 right now but I think they did away with relays and use triacs to turn the coils on/off. With it being so sporadic I would also be looking for loose connections anywhere in the junction box on the tank and controller.
I had a newer mce 2000 that the car was trapping people going down once or twice a week with a valve limit timer. Looked over status of all switches etc when it would happen and everything was normal. Ruled something out each trip, started by cleaning valve screen, limit switch contacts, etc. Ended up swapping the two up with two down coils and rewired in the j box on the tank, if problem was a coil it would change to a motor limit timer. It fixed it, it was a bad connection on a down valve coil wire to the multi conductor in the jbox. Which I had already checked and looked good actually.
Don't take anything for granted.
