I have a LP gas, upflow furnace with a standing pilot. I can turn it on, the pilot lights the main burners as normal, the flame is blue like it should be, and the blower comes on like normal after the unit heats a bit. The problem is every 5 minutes it pops a fireball out the front (I now have no hair on my left arm!) but keeps running as normal after that's. This is only during a burn cycle. I cleaned it out just to make sure it wasn't clogged, but the problem didn't go away. I sprayed the gas valve with soapy water as it was on and burning but didn't see any bubbles. It has not tripped the "flame rollout switch" either, which is disconcerting. Needless to say it's off now!
Thanks,
Mark

Mark are all of the burners firing. One may not be firing and raw gas is pouring out and igniting. When the blower fan comes on do the flames lift off the burners and move or "dance".. A cracked heat exchanger would cause this. This could also cause a burner to go out then dump gas and then ignite it. If you check this be careful so you don't loose more hair or worse. Use safety glasses etc..

As Pelle said, bad or partly plugged heat exchanger. A dirty or rust burner. A gas leak around the manifold or gas valve. Gas pressure (output).

A partially clogged pilot turned out to be the problem, so the furnace would turn on the main burners and it would take a second to light them, all the while a significant amount of gas had built up, causing an explosion when it would light.
Go figure!
Thanks,
Mark