We too have the tankless water heater on our Greyhawk. When we bought ours, it was equipped with the first generation model. It did not have the adjustment knob that the Gen2 has now. It was simply a high/low switch. We have now switched to the Gen2, and have really enjoyed it. What you are experiencing sounds like limiting. If the temp is set too high, the high-heat sensor will trip and shut off the burner. After a few seconds, the water and sensor will cool, and the burner will reignite. The cycle will repeat itself again and again. Try turning the knob down to a lower temp setting. Jayco was useless when it came to resolving my issues with the original heater. Do yourself a favor and speak directly to Girard. They are very helpful and want you enjoy their product. It's possible that your problem is totally unrelated to this, but the issue I described is very common.
Brian, Mary, kids, beagle - Moose, and mutt - Maggie.
2012 Greyhawk 31FS
2013 Ford Explorer Toad