Rheem model #GP40T6PF00 lp 40 gallon hot water heater Will not light all the time. It may work fine for a week or two then when you least expect it no hot water! the light is flashing twice? when I turn the power switch by the fan off then back on the blower will start and you can hear the ignitor clicking but it takes a long time to actually light sometimes, sometimes lights quickly. What to do?

Water Heaters - Rheem - 40 Gallon FVR NATGAS Low WATER HEATER 6YR 22V40SF