After a lot of looking I realized it was due to a poor electrical connection. The lead wires are connected to a tab, and that tab makes contact with a tab on the braid. The tab on the braid has a small protrusion, and it looks like that should mate with a punch-out on the lead wire tab. But on mine the punch-out wasn't punched out:

I think the design may be OK, but it was poorly manufactured.
Has anyone else had similar intermittent electrical issues? Did you punch-out the section on the lead wire tab? Or did you find a different solution?
Thanks,
Ron