Willie and 103west43rd:
That was exactly the problem!
The upper contact tab in the housing had been somehow knocked loose.
The solution I used was a dab of hot glue at the base of the contact leaf after hand-positioning the contact back into alignment. Since it was the upper contact I was working by feel only but managed to secure the contact back into place.
The result is - IT WORKED!
I drove it on the Interstate for 70 miles (hey, it's MISSOURI!) in cold weather with zero failure.
Problem solved. $450 not spent.
Thanks all for your replies!
