Those were times you were in suspended animation. Check for signs of alien abduction, addition of implanted tracking or monitoring devices (any new and interesting bumps or lumps?), sudden weight loss suggesting missing internal organs (are you SURE you still have two kidneys?)?
Have you developed sudden cravings for jalepenos and ice cream
, and does arguing Shimano vs Campy vs SRAM still have the appeal it once had? Do the moments when it stopped recording coincide with the weekly office status and team-building meetings or diversity training (those Shimano/Campy/SRAM folks are people, too)?
Also re-read the instructions for your HRM - does it have a manual start/stop setting?
Any proximity to strong EM or RF fields? My Polar 720 (even with coded HRM signal) flakes out at some places at work and at one or two spots near electrical substations, and there is something in my car (but not wife's or son's) that triggers the speed sensor autostart.