Hi all,
Is it an old or new Polar you are using?
I've had similar high spikes with an old Polar FT1 HRM when cycling outdoors over the last couple of years. The problem does not corelate with reaching the top of a hill, and has occurred even just freewheeling downhill. I also use a cross trainer indoors with the same setup but have never had this problem, despite heart rate of up to 150. So for outdoors I used Sonogel electrode gel and shaved hair to ensure a good contact. This stops the area drying up, especially in warm blowy weather, but the problem continued. So, suspecting faulty electronics aggrevated by outdoor conditions (vibration, orientation of watch to strap, heat...), how to test?
I've tried changing the watch battery and checking all internal contacts. I've also replaced the battery in the Polar T31 chest strap- this is not easy as it is sealed and you have to heat and cut the back off, but there are youtube videos to guide. You can buy "new" T31 straps but beware they haven't been made for 10 years so the internal battery might be expired. The problem continued.
The Polar strap transmits on 5kHz, now regarded as old school. Most modern stuff uses Bluetooth (for mobiles) and/or ANT+ (I believe for newer gym machines).
All I need is a self-contained HRM reader, so I recently got one of these for about £40: Beurer PM25 Heart Rate Monitor Watch:
https://www.amazon.co.uk/gp/product/B000I6FN5I
The kit is similar to the Polar, and it comes with a bulletproof strap also using 5kHz. Either strap will work with both watches.
You can only wear ONE of the straps but you can carry both watches on your wrist or arm and compare. Note that the Beurer might be tolerant to slightly irregular heart beat, so it might not register a real medical problem and the whole exercise might in the end not prove anything.
I'm only at the start of testing the devices. Two rides so far with the Beurer strap, no dropouts. I'll re-post when I have more results.