I would check out the 24 volt power supply that supplies 24D and 24C if this voltage gets below 20 volts then you can have some strange problems. I looked at the board prints for top and bottom, and intermediate cards that control car and hall calls, the only thing that is common is the 24 volt supply, the 5 volt supply and the common, I would check the 24 supply first as this is most likely the problem. there could be a problem with the 5 volt power supply but i dont think it would give this problem, check 5volt on the power board from sc to sp test points on the front of the card.