I have an Armstrong Ultra II gas furnace and a Lux 1000 thermostat. I had this same problem with another thermostat and I installed the Lux 1000 and it worked fine for about six weeks and now it is starting to do it again. It is an intermittent problem. Any thoughts?

Look for a shorted wire in the thermostat wiring from stat to furnace. It can cause a intermittent situation like you have. To test unhook the R wire on the furnace itself and the furnace should turn off.