Atwood 6 gal gas water heater problems?

Took the board to another RV shop, same result, tested OK. Bypassed the thermal cutoff (which also was new). Replaced the igniter probe, cleaned every connection(many times!). All with the same result, Lights, runs a few minutes, shuts down. Shop fellow suggested lifting the safety valve lever to see if tank was actually coming to cutoff temp. No, luke warm. This would suggest a blockage if the unit had a bypass valve, which I checked, it doesn't. Went ahead and ordered a Dinosaur Universal igniter board, small size. We'll see what happens! This after just replacing the cooling unit on my Norcold fridge....was that a backbreaker, getting that thing back in the coach!