I had a 510, and it would also reset any time I was away from it for more than five minutes.
It was also quite easy to restart the 510 accidentally after you had stopped it at the end of your exercise, resulting in your exercise file being compressed and no longer worth downloading.
I changed to the 720, which waits 30 minutes before turning off, and which does not compress old exercise files, although it is much harder to accidentally turn on.
I think Polar probably realized those were a couple of design glitches and changed them in later models; although the more expensive models may also have a slightly bigger memory, I'm not sure.
I complained to Polar about all this, and they told me that there was no real solution to those particular problems.
Thanks a bunch, I bought my new one on E-Bay so they wouldn't make so much off me.
whit