Polar HRM stupidity...

On my long run today, my Polar HRM was working fine...until mile 5.

Then it went nuts. HRM logged in at 170-200 (usually long run HR between 140-160). So I slowed to a walk and within a second or two, HRM logged 138-140. Start running again, in a minute or two, 170-200. Stop, back to 138-140. I was not working hard enough for it to be 170-200 - I've been monitoring HR for a couple years and I'm pretty OK with knowing what my body is doing.

Swings from 138 to 200 within seconds, back and forth.

Then from 200 bpm to 0. Now I know I'm not dead, so that can't be right.

I stopped, readjusted HRM strap, and it seemed to work OK for the rest of the run.

Any ideas to avoid this stupidity in the future?

It didn't happen from the get-go, so I don't think the problem was conductivity.
It definitely wasn't related to my effort.
I have no sense that my battery is not working well on either the strap or receiver. Other efforts (e.g., my bike ride on Friday) did not have the same problem.

???

Thanks!