Attwater 6 gallon water heater gas-elec.

With mine doing something like that it was the air shutter on the main burner tube at the orifice end. It was too far over leaving the holes too big. Slide it to make smaller holes.

I just had a thread about that--go to the last few posts in that thread.

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29830330/srt/pd/pging/1.cfm

https://www.amazon.com/Atwood-91638-Main-Burner/dp/B004QMXSXY/ref=sr_1_37?ie=UTF8&qid=1548807628&sr=8-37&keywords=atwood+rv+water+heater+parts

Dirty burner tube
Dirty gas orifice
Dirty combustion chamber
and last but not least....LP System Pressure