I have a heil 9000 furnace. 20 below zero here last night, house was 52 @ 4am. I tapped on the limit switch and furnace kicked in. However it limited out frequently until reaching 68. Now it seems to be keeping 69-70 but when it shuts down after reaching temp the thermostat blacks completly out for a period of time. Does this mean the limit switch is shutting everything down for a period? When the furnace kicks back on the room temp can again be seen on the thermostat. I have had limit switch problems with this furnace for 15 years. Any suggestions? And can this switch be replaced by do it yourselfer?