OK, I think -- I hope -- that I have my answer.
I inspected the control system and heater tonight.
To my surprise, a 10 ga (at least) wire leading to heater was HOT to
touch. (and carried voltage and current) I measured current, it was
I found that the crimped connection of a 10 ga wire leading to a
ring terminal screwed on the heater, worked itself loose.
I recrimped the connection (kept original ring terminal, just crimped
After that, the measured current increased to about 12A, which, I
think, is about where it ought to be.
If that turns out to be the solution, I may replace the existing cheap
wires with my beautiful 10 ga nickel plated, 200 degrees C rated
military aerospace wire, of which I have a big spool.