have a gas forced air furnace. not sure of the name brand. when it calls for heat, the burner kick on, but won't stay on. they will burn for a short period of time, then they will turn off. the fan motor stays running, but i had to turn the start temp for the fan way down so it would blow what warm air there is in the unit out. it just keeps repeating the cycle. i put in a new fan limit switch and it does the exact same thing. my next step i think is to get a new flame sensor. if this does not fix the problem, i don't know what to do after that.

have you cleaned the flame sensor that you already have?