This flame is not likely going off on a high limit due to clogged filters as suggested previously because the unit goes off on safety before the blower comes on. If it works sometimes and not others it is most likely a flame sense issue or an electronic problem. If the boards logic tells the inducer to keep running if it thinks the flame is lost this may be your clue. You can attempt to find the red LED trouble light on the circuit board(or in burner compartment) and this may tell you exactly what the problem is but you may need to tape the door interlock switch so you don't loose the code when you open the blower compartment. You can clean flame sensor with very fine sandpaper or steel wool but replacement is usually the best.

Check the filters first, then check for blockage in the return or supplies. An easy way to do this is by putting your hand in front of the vents to make sure accurate airflow is there. Your furnace has limit switches on it that detect heat, and if heat is getting trapped inside the unit to much, you will trip these switches and the blower will just run forever. Then you have to reset it or wait forever. But try those few things first. Hope this helps.