roof furnace that won't stay lit
 

I'm working on a Payne roof furnace. I've replaced all vital parts and checked the gas pressure. The furnace has a johnson control ignitor and dual valve. Once the burners ignite, they go right out again. Sometimes it will stay lit and then the furnace works until another cycle. Then I never know what to expect. Short of replacing the entire valve and ignition assembly, any suggestions?

Welcome John:
I would check the heat exchanger closely, you may have a burnthrough or a rustthrough. If that is the case and the furnace is over 10 years old, I would consider replacing it because installing a new heat exchanger is very expensive.
Glenn