Removed OTR microwave. Checked the thermostat switches while power was applied with a DVM. Noticed that when a certain set of wires was moved the slightest bit the microwave would power up. Traced the wires to the thermostat switch for the cavity. Noticed that where one (of two) Faston terminals was crimped to the thermostat switch was black. (should be copper colored). That is a sure sign of arcing from a loose connection. Since the specification for the switch was to trip at 248F degrees & it's the first in a series of 4 switches, decided that it would be OK to solder the wires together until the part arrived. Any other switch, I do not recommend temporarily shorting due to safety reasons. Replaced the switch and directly soldered the wires and applied 3/16" heat shrink. Problem solved! Intermittent problems can be a challenge. When power is applied, stay way from the HV ckt (that's the big transformer and capacitor)!!! Short the HV Cap to ground w/o power applied before servicing. Check the thermostat switches first, there are 4 of them.