said by seandrom :
Furnace is at least 7 yrs old. Any suggestions as to what the problem could be? Electronic or just a bad filter unit?
Our furnace is much older with no electronic filter but ... a few years ago something like this happened.
The HVAC guy came and changed the flame sensing element (aka thermocouple) and charged me $$$. If it happened again, I think I could do it myself.
I am curious how you are sure that the thermostat is working properly. Have you actually checked heat call relay tripping by adjusting the temperature settings?