Thermostat indicates that heater should be turning on. Circuit breaker has power, tested it. Cycling one of two circuit breakers starts Coleman Electric heater operation. Sometimes this will cause heater to work for days. Then out of the blue the house gets cold and the breaker has to be cycled again. Could it be a week low voltage transformer or one of the relays I should be testing?

It sound like you have a heat sequencer going bad(if it all electric and not a heat pump). The sequencer times on and off the heat strips and blower. Technically it's a time delay relay.

That is what I was thinking but a low voltage transformer that is not outputting the correct or a lower voltage that the relay might need to start the sequence when receiving a bust of energy (cycled CB) will set it off on it's way. I'll attack the relay's first then.