Yesterday, I left the chest strap at home and went to the gym. I entered a warm-up spin on bike for ten minutes, worked on strength, then went back to the bike to cool-down. During the cool-down, my Garmin 935 optical HR monitor reported an HR of 235 bpm at the start of the activity. I cancelled the activity, restarted it, and it again reported 235 BPM. I cancelled the activity again, powered down the 935 from the menu, started it back up, and again, the HR monitor read 235 bpm. I repeated this process several times to no avail. I was on the verge of concluding the watch was broken, and after about 10 minutes the watch began to report a reasonable heart rate of around 60 bpm.
Today, I tried to download my structured workout from training peaks and the 935 kept reporting "BLE timeout error". I don't know what that means. I gave up on downloading the workout, and went for my run without the structure. When I came home, the 935 would not automatically sync with Garmin connect as it normally does. I went into the 935 setting menu, disconnected and reconnected the phone, and it finally synced.
My concern is that all of a sudden the 935 has become glitchy. I think it may be because I plugged it into my computer and it updated a few days ago, but I'm not positive. Nothing else has changed. The unit is only a couple months old.