GE microwave error - PROBE

Question

I have a built-in GE microwave, circa 1994. Never had any problem with it until now. The display reads PROBE and the touchpad no longer responds. If I switch off the power to the unit for a day, the problem is corrected and the microwave operates normally for about a day. Then PROBE reappears on the display with the same unresponsiveness of the touchpad. Any ideas?