My issue was/is Edge 800 related, not Edge 510, but it's all Garmin, so....

At one point in the Summer of 2015, when syncing to Garmin Connect, I get an alert that said an update was available for my 800. Despite the horror stories I've read here and on other cycling forums, I foolishly did the update. My 800 was never the same after that. Constant freezes and random shut-offs while riding and when the unit didn't freeze up or shut down, over half the full rides I did were missing information when the sync was complete. I think I'll be using my smartphone exclusively this year when I need to / want to record ride data.