1) Could be the vapor sensor. That's a suspect.
2) General troubleshoot: clean burner and combustion parts, clean air intake, bleed air from gas line, check gas pressure. Color of gas flame should be clear blue, and flame should be strong, and no combustion by-product dripping back into combustion chamber. No chemicals of any kind stored in air supply. Plenty of fresh air. Water heater in hot attic needs more air. If water heater has igniter, then igniter has to be checked. Flame sensor must be positioned correctly and operational at time of combustion, or sensor will signal gas control to turn off gas. Gas control valve can be bad.
Open following link for possible illustrations and reading.http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
3) Specific troubleshoot begins with identifying gas control valve and reading the service manual.
I think the Rheem/Richmond power vent has intellivent gas control
Open following link to identify gas valve and download possible service manual:http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html#intelli
4) Take a moment and rate answer:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.