I've never had this before so I was a bit freaked out by it. I was happily spinning along on the trainer tonight keeping my HR low - mid 150s after an hour with an hour to go - when all of a sudden I felt my chest spasm a little bit. It didn't last long - 5 seconds at most but long enough for me to check my HR which was showing 238. Couple of seconds later it went right back down to 152. No gradual descent - a complete drop-off down to 152 where it then stayed for a bit before slowing moving up to the 160 range by the end of my workout. It wasn't a monitor malfucntion as I checked my PowerTap after and it had caught the spike too. In any event I "felt" it - like that part of my chest had an old-style alarm clock going off in it! During the racing season my HR never went above 195 and the few times it hit 195 I thought I was going to pass out and/or puke so 238 is very odd. Checking the PowerTap readout after downloading it I saw that my HR was at 150-ish and then dropped to 130-ish for a full minute before the big spike. Very odd especially as there was no change in cadance or power output. Both held steady at 90+ and 185-ish respectively. Anyone had this themselves?