I have a Navien CH-240 Combi (new install) in my second home, and the domestic hot water temperature is unstable. I have the temperature set to 120*F, and when I open a faucet, the boiler ramps up, the water gets hot, but then the boiler cuts back and the water gets cooler, and this continues. The water never really gets to 120*, and the temp goes up and down and up and down...can't even take a shower.

Anybody ever seen this happen and have any idea what causes it? Tech support had some ideas, but we haven't gotten to the bottom of it yet. Been through the dip switches, etc, tried running with and without the remote controller, but can't seem to get it working. My plumber and I are kind of tag-teaming it, but we're not getting anywhere, so if anybody has experienced this, I appreciate any help. Would a dirty flow sensor cause this? Or low gas pressure?