My heart rate monitor apparently believes I'm super human... or dead.

I've had a Timex Iron Man w/ HR for a few years now, although it's been out of service with a dead chest strap battery for the last year or so. Recently I got a new battery for it and figured I'd start using it for some targeted training. You can set HR zones based on your estimated max HR and then let the watch remind you when you leave that zone. All seems fine and dandy, if it were only so easy.

Today I decided to get in an easy run, the first since my slight muscle strain from Nation's, and set myself up to keep my HR in zone 3 which is 137 to 156 bpm. Generally my normal running puts me in the low to 160's so I'd be going a bit easier then my normal pace. For the first mile it was going alright with me going over occasionally, generally a beat or two per minute. The next thing I know I'm rocking a 215 HR and the darn thing won't shut up. After about a minute of constant beating I gave up on it turned it off and proceeded to finish up my lunch run. Of course that means I ran the rest of my run significantly harder then I planned and my legs are now nice and tired for my ride tonight.

I'm curious as to whether this is just my HR monitor being a piece of junk, or if maybe I should be worried about a heart murmur or something. Maybe of course I really am just a super human with a max of 240 bpm like my monitor is fond of reporting to me but somehow I think that just isn't it. But seriously, is there anything to my fears here or is it really just a piece of &*^@! (horse poop).

The rate, even when seemingly working will jump around quite a bit. Say from 167 to 150 and back and even after telling me I was rolling at 215 for a few minutes, it next said something along the lines of 145. What is this all based on anyway? Beat to beat? The number of beats over the last minute? The last 5?

I guess maybe its time to start thinking a bit more seriously about one of them GPS/HR watches...