This may not be a thermostat problem. When was the last time the furnace was serviced? Check the filter and see if needs to be changed. If you have a dirty or restrictive filter, the furnace could be experiencing a temperature limit override. This is a safety feature of the furnace so it does not overheat and damage the heat exchanger. Poor air flow can cause this problem.

My fridge doesn't get warm on top, or barely so. Mary, I use heating cables rather than mats, and always have so can't offer much personal advice. However, I believe that the mats are intended to be effective without the use of an additional thermostat UNLESS directions indicate that a controller is required. My suggestion is to visit the websites of the manufacurers directly and find a customer service phone number or website in order to get your specific questions answered. They are probably most efficient when used with plastic flats that many of us use for germinating seeds. The standard flats measure somewhere around 10X20 and mats are sized to accomodate them. I use two or three flats to germinate what I (and a couple of neighbors) need. My cable has a built-in thermostat.

Capacitor is simply a wild guess,we are operating here without much detail you know. There are other possable reason's beside's caps. To name a few,extremly dirty coils,fan turning too slow,fan blade in wrong position,wrong fan motor,wrong fan blade,high resistance electrical connection,bad/clogged metering device and of course bad compressor. I don't suspect compressor because they usually pull more than 50% extra amps when going bad. 30 amps probably mean's it's getting off the start winding because start pull's more like 100+ amps. If the small cap is 3 to 7uf it's for the fan. If you can't find exact replacement cap,20% over in uf and higher volt rating is ok. You still didn't tell us how long "good while" is.

I think it is working. I was encouraged by the temperature being so even all day. But it must be the lights that are keeping it just warm enough so it doesn't run all day or not very often anyways. Tonight, after the lights shut off at 7, I came down at a quarter to 8 and the readout showed 69.9d f. and the heating light was on and running. So without the lights the temp dropped quickly and heater came on. if that keeps up, I'll be satisfied.