Just talked to Garmin today. They said the most likely source of the high readings is that the connections in the strap are worn out. They are sending me another strap, free of charge. Nice folks.
I figured it would be that or a salt bridge that was short circuiting something. I'll get in touch if it keeps happening! Thanks Charlie!
Well, I'm sure mine could be a salt issue. Washing it periodically is good maintenance, but mine got beyond that.