Probably your gas meter and or the gas regulator B4 it are failing from old age/use. It is rare but they do fail and are mechanical devices. Once a year I get one that fails and I had a job that took me 2 callbacks B4 I caught it. Furnace would not keep running but started OK and also the water heater pilot went out daily. Talk to your gas supplier NICELY and don't acuse them of anything or they will blame the furnace. They replace meters every few yrs where I am as part of a weights and measure accuracy program. The gas regulator can fail too.