Does it re-fire every time for a few seconds, or do you have to do something to get it to restart?
It could be that the igniter needs cleaning and/or adjusting. It's part of the flame sense, so if it doesn't sense a flame it closes the gas valve after a few seconds.
It should have a 1/8" gap and the rod should be clean. Fine emery cloth usually works.
You can find a service manual online for these heaters.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!