Water Heater Ignition Problem

Hi all. I've got a 1990 M23 (3023). Love our TM. But... The water heater is not lighting properly - all the time. ??? Sometimes it fires up fine, others not. It's a 3 gallon gas only model. Now the problem.

When the heater lights the main burner, the gas sometimes burns in the main burner tube. I have taken everything apart, cleaned, reassembled, all for nothing. The air mixture is OK, I have checked that multiple times. When the heater fires in this manner it burns very rich, you can smell the sooty smell and it is depositing soot on the side of the TM above the heater exhaust grate. When lit, it makes a high pitched sound as the air is pulled in wrong. Remember, this problem is intermitent. When it lights normally it works great.

I'm out of ideas and thinking of replacing the gas valve assembly. Anyone out there have another idea?

Thanks,
Camperboy