I have an intertherm furnace. When in AC mode it will run a while and then the furnace will shut off and the outside unit will continue to run. At this time I also will have the "the flame fail light" blinking continuously about the power light. I have replaced the thermostat,wire to the furnace from the thermostat and the circuit board. I have had a service tech her a couple of time but it seems not to do it when he is here. What else could be the problem?