Furnace won't stay on. my intertherm furnace has been working very well until just lately. I woke

Question

my intertherm furnace has been working very well until just lately. I woke up one morning and the it was 61 degrees in my home but my thermestate was set for 68 degrees. I noticed that my furnace would still blow out warm air but only for 20 to 30 seconds before stopping and then starting again. The furnace would run again and then the fan would run again for the same amount of time...and so on.. Basicly it wouldnt stay blowing long enough to raise the tempature.....