Sometimes all 5 burners will light and heat will come on sometimes they won't light and heat will not come on

Is your furnace a newer one with a forced draft and control board? If so, it will not proceed to ignition and the gas valve opening until the pressure switch signals the board the draft blower is running. Check the blower and the pressure switch.

If the gas comes on, and goes off in a few seconds, it is the flame detector.