I'm with tatest on this one. We are all just guessing based on experience but if it really is running for 30 seconds... as in the burner lights and begins heating... then the igniter is working. The gas valve is being opened so the circuit board is doing at least part of its job. The other part of its job is to shut down the LP if it did not light so you don't fill the bay with LP. It's acting like the flame detector has failed. The board is not receiving the flame signal so it is cutting off the gas for safety.
Personally I would replace the flame detector which is a thermocouple first. It's cheap and a likely failure point since it is exposed to a direct flame often. If that didn't fix it, I would replace the circuit board.