I frequently will see my HR jump 30 -35 bpm when doing hard intervals or in races. The first couple times I saw the jump on the monitor I panicked and felt a difference in my chest. My Z4 HR starts around 148 and Z5 begins at 158 with a maximum around 170. I could be doing a Z4 interval and at 153 bpm suddenly the HR jumps to 180 bpm and will stay in that range and reflect any change in intensity by increasing or decreasing. As soon as I begin to recover after the interval or coasting down a hill it will usually drop back to 146- 148 bpm as quickly as it elevated. I think the highest it ever recorded was 193 while doing some high cadence intervals on the trainer last winter. I have taken screen shots of the my HR during workouts where it did the jump thing to my doctor and he seems OK with it for now since the HR reflects my efforts then drops back as soon as recovery is introduced. He told me that it could be an age thing and to just keep an eye for trends. Luckily I do most of my riding and training with a power meter and the data retained to my laptop. I use three different HR monitors, one for the Power Tap, one for the Stages Track Crank and a HR monitor only for mtb riding and the same results happen with all.