I am having the same problems as described in this post with my watch, but it has only been happening since I updated to watchOS 2.0.1 a couple of weeks ago.(prior to that I had remained on watch os1.0.2) It would occasionally happen in the other OS but not enough to bother me. Since updating, the result is that all total my calories burned when I'm doing a workout involving high intensity interval type training (like Jillian Michaels Body Crunch, or Insanity etc.) are lower. I've noticed during the workout, that the more I sweat and the higher my heart rate gets, the more this happens. Like others have posted here, it seems to divide my heart rate in half. It seems to be the worst as I get above 170 bpm. Perhaps I need to recalibrate? It is winter time here in the US and I haven't been outdoors as much, nor have I done an outdoor workout since I updated. I will say, with the other OS, all my activity app accuracy seemed to get better the more I walked and ran with my phone. In fact, it was so accurate, that when I didn't have service last summer in remote parts of Colorado, it was still dead on in the distances I had hiked, without even having my phone on me. (Couldn't count for elevation of course, but that's another story) The thing is, I lost 15 lbs after getting the watch and using the Lose It app, along with my watch. By following what the app predicted I would lose each week by netting so many calories each day (getting the readings from the watch and doing the math on what I was burning and logging etc) all the data was dead on correct and that is EXACTLY how much I lost each week. I feel that this is a pretty good indicator that the readings I was getting before the OS update were pretty accurate. To me, this means it probably is a software issue and that hopefully they can fix it. It's pretty frustrating to be working out at a high intensity and have your heart rate be 78 bpm, when the your hear rate was previously measured at 144 or something a minute before. I AM going to recalibrate with my phone and see if this improves the readings.