Question Re; Girrard Tankless HW Heater

I installed the Girrard into my 1972 Streamline Imperial last spring and all through the spring and summer and most of the fall, it worked great. But now that the weather has turned cold (I'm in LA, so that's a relative term, 40's at night) the best I can get is luke warm. I suspect it could be a gpm/flow problem because when I turn on the cold water it gets almost hot for just a moment, then goes back to luke.

Any suggestions on what could be wrong, or a fix?



Thanks, Patrick