Suburban water heater (SW6D)

Yesterday, after not being used for several months, the water heater in our camper started up as normal with a nice roaring blue flame but after 3 or 4 minutes and before the water reached temperature, the flame slowly started to die down and tuned yellow. The small yellow flame then gradually moved back through the burner tube to the gas orifice. Turning the inside power switch off and on just resulted in the same small yellow flame very close to the gas orifice (no visible flame at the electrode end of the burner tube).

The water heater power remained off overnight but today without me doing anything to it, the heater functioned normally when powered up (nice roaring blue flame) and it continued to work properly through several heating and cooling cycles.

There is 12 volts at the gas valve, no obstructions in the burner tube, the electrode is clean and the gap properly set at 1/8 inch. The propane tank is almost full and the stove, furnace and fridge work fine. The water heater is 15 years old and has been used for at least two months annually.

Any thoughts would be appreciated.