[AmeriServ]

Sounds like the stat may be set up wrong or going bad. The night set back option should not make it short cycle, which is what it is doing at night. The furnace has nothing to do with how long the burners fire unless it does this all the time. The stat will allow the temperature to drop below the set point (usually called the differential) usually 1 deg F before it calls for heat and it usually will shut off the heat at the set point or just below. Sometimes people set the differential too low (at 1/2 deg F) and this causes short cycling.

I would just replace the stat, you can get fairly inexpensive programmable stats these days and I am more than sure the problem will go away.

BTW, it is not a great thing to short cycle the furnace as it adds like cycles to the unit. All parts are made to last a general amount of cycles (on - off) including the gas valve, ignition, heat exchanger etc.