I have a Hotpoint side by side and the coils keep freezing up. Initally freezer was fine but refrigerator was too warm. Called a repairman out and he replaced the sensor in the bottom of the freezer stating if that didn't work it would be the mother board. It didn't work so I replaced the mother board, coils still froze up so I replaced the defrost themostat at that point almost 2 weeks ago. Took panel off today to check and coils are freezing up again. Removed the thermostat to check it and the digital meter I had seemed to jump around on numbers occassionally stopping around 21.9 or so, also noticed a small amount of black powder substance coming out of the thermostat when I removed it. Should I replace the thermostat? Is there any other things I should check or it could be? Thanks