10yo Rheem water heater RTG-74PVN-2. Often when a tap's opened the water heater kicks in briefly, water's lukewarm for 2-3sec, then cold. Sometimes there's hot water for tubs and shower. Error code 12

SOURCE: how to correct error code C-7 on my RTG-74PVN Rheem Pronto

I had the same problem and called the manufacturer. The error code C-8 was followed by the number 76. They told me to disconnect the thermostat/digital display from the main unit. This was fairly simple to do by removing the front panel. The unit works steady now without shutting down after a few minutes. The help desk told me that the main circuit board needs to be replaced for the unit to work with the display and that they would mail me one asap. At least we have hot water again.

Oakville, Ontario

SOURCE: I have a Rheem rtg-74pvn-2

Before calling service technician, open following links for resources including Rheem chat and possible manual:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/No-Error-Code.pdf
http://waterheatertimer.org/Crossover.html

Typical tankless manual says to have yearly service which includes checking household plumbing, gas supply, vent and condensation, check sensors for loose wires, clean and inspect burner parts and ignition factors etc.