Gas Furnace heats but not for long?

I have a gas furnace thats about 8 years old. During the current midwest cold snap where temps have been in the teens, the furnace won't heat the house to desired temps. It will turn on, ignite and provide heat for a short period of time and then turn off, after about ten minutes it repeats once or twice and then shuts off for an hour or so. I have replaced the thermostat and the filter. I see a flashing light which i understand is a diagnostic code and it appears to be flashing 3 or 4 times. I have read several possilbe troubleshooting solutions which say to either sand the flame ? or clean off the burners or? I can't afford a repair that i can do myself. Any advice or help would be greatly appreciated. Thanks Dave Rimmer