Originally posted by Maxt
Flame sensor.
http://www.youtube.com/watch?v=Uly51j6oT9s
Also on those gas valves, the connection plugs are notorious for shitty connections... A dental pick works to spread the pins to tighten the connections.
I had similar symptoms on my furnace early last year, disconnected the flame sensor and took an sos pad to it, reinstalled and has been fine ever since. ymmv
You start with a bag full of luck and an empty bag of experience. The trick is to fill the bag of experience before you empty the bag of luck.