Mine did the same, turned out to be the control board.....
Atwood, on many devices (I know furnace best) uses a single wire flame sensor/ignition system.. During Direct Spark Ignition phase it's hauling aroudn 1,000 volts A/C (Based on the size of the spark gap) See note on that.
Then it switches to flame sense it's looking for less than 1/2 volt (480mV)
Imagine if the switch does not go properly, the half volt senistive chip is hit with 1414 volts (Peak voltage for 1,000 AC) and .. Pop goes the flame sensor chip.
I think that is what happened to my furnace.. So, it seems, does dinosaur boards cause on the replacement control board they put a nice little gas discharge tube in the ideal spot to deal with just that issue. I am assuming that is why it is there.
On my Water heater, when it did what yours is doing, New control board fixed it.
NOTE: before throwing money at it, make sure of the following:
1: You have propane
2: Do a Full PM on the burner (make sure the oriface is not clogged by spider web)
3: Check the spark gap, Overly wide spark gaps are common in DSI devices it seems.
