Here's something very obvious I never thought about! :-[ If you're doing a long ride or race & you stop for a break, leak, fix a puncture etc. After a certain interval, about 10 minutes I think, if you have moved away from the bike, the watch, mounted on the bike, will stop receiving a signal from your chest HR transmitter and will auto switch off out of measure/record mode & back to "time of day". Then you must restart as a "new" ride! Very obvious when you think about it!! : So either wear your watch on your wrist or stay within a couple of metres from your bike.