Man, you guys are fast! Thanks for the ideas.

The vent is higher but not by much, I had extended it a few inches
when we first moved in because draining the sink could sometimes cause
gurgling in the shower p-traps, which stopped when I extended the air
gap pipe 15 years ago, not much room to add more and be able to screw
the thing on.

The heating concept is an interesting one I will have to pursue. I
started to say that wasn't the problem, because I always use miser
wash (no element) and air dry. But as I was getting ready to type
that I remembered that my wife turned it on tonight, and hmmmm does it
only do it when she uses it? maybe.

Thanks again for the thoughts, I'm going to try a run without heating
followed by a run with it and see what happens - and do it at night
like now when the temp has dropped suddenly.