I have a Janitrol Furnace Model GMP 075-3 Rev B. There are three flame jets.
The first flame jet has the ignitor (HSI) and the third flame jet has the flame sensor.
When the furnace is triggered by the thermostat, the first jet will ignite,
Just fine and the other two will flicker on and off, usually after few tries or after it warms up all three jets will stay on and the furnace will work as it supposed to. The
Furnace will try three times and will then have to be reset. I took the pipes part that feeds the burners looking for obstructions, I switched the burners around, I bought a new gas valve, replaced the old gas valve and the problem still exists.

You could try cleaning the flame sensor. Good luck

The flame isn't hitting the sensor long enough to trigger it and when it does the sensor works

Just trying to help, here's a link to the service manual. Good luck

http://www.hvacpartsshop.com/Gmph075-4%20Series.Pdf

Thank you for your help