Sorry I can't help with your issue, but I have a similar problem with my Polar 510. Maybe the 510 has quality problems?
A couple of weeks ago, the stopwatch feature on my Polar 510 began to stop in the middle of exercising. If I hit the red "Ok" button, it would restart. It happens both on the road and in my basement on the trainer. Sometimes it happens a lot and other times only 2-3 times per ride. The monitor is only 6 months old (only used for 3 months).
I know that is a bit different than the malfunction you are experiencing.
I don't see how my problem (or yours) oculd be related to any of the usual interfeences like too loose of a strap (mine is tight), issues with the electrodes (the monitor keeps measuring my heartbeat the whole time with no problem), electrical interference, or static electricity from my shirt brushing against it.
Anyone else have a similar problem?