Flame rollout?

I have a forced hot water system that is experiencing flame rollout. I checked the draft up to the stack and it seems to be clear. This unit is running in unison with a hotwater heater. they are Y'd off the chimney. the heater is running fime. I know the gas was off for sometime, I was wondering if the turning on of the gas might have moved sediment into the main gas regulator valve and b causing this. The flame runs well for about 3 minutes though. Could someone please give me any and all possible causes for FLAME-ROLLOUT! THANKS!