I have a gas furnace approx 10yrs old. Within the last 2 weeks I have noticed that it will go off and on. The thermostat will be set at 70 and the temp will read 68. So it will try and come on then goes off, tries to come on then go off 3-4 times. Sometimes it will eventually kick on and work. Other times it goes through trying the doesn't come on at all. Sometimes I can turn the thermostat of then back on and it will work but not always. And it doesn't do the on and off thing all the time. Any help would be much appreciated.

Try cleaning the flame senser in the furnace.