When a furnace starts you hear the vent start and it keeps going to till the end of the cycle. My vent starts then stops and repeats this process while running through a cycle. The burners shooting out gas are still running when it does this. Help I don't understand what is wrong. I recently replaced the flame sensor and it worked perfectly for a few days now its acting up again. When I go up to the furnace I notice the wire connecting to the heat sensor is lose, I re-tighten it and it works perfect again. Then a few days later it acts up again help I don't know what's wrong.

Check your vacuum hose for holes or vacuum switch