White-Rodgers 1F85-275 thermostat shows 1st and 2nd stage and I need only to set it for normal use with no special programming. Furnace won't ignite when I raise temp and setting drops down to 60 on its own at some point. Peadon heating claims problem in thermostat and not the furnace. They do not want to do a free service although they said it is under warranty. How do we fix this?