Ok, I have a new FSA Powerbox, basically a re-badged Power2max. After a few rides the Garmin flashed up a "low Battery" warning for the FSA. When I checked the battery status in the sensors menu it said the battery was fine. Anyway I changed the battery but 2 rides later the message popped up again.
When I checked back through the power data on my rides I saw the following dropouts. This is just a small sample, there's loads. I happens indoors too, I did a Tacx VR ride and recorded my power meter on the garmin while recording the Tacx powermeter directly on the laptop. Got the same results, PM dropping every minute or so but no problem with the Tacx.
I recorded a ride using th PM on Zwift, no dropouts.
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I've turned off bluetooth on the 520 and had my phone on aircraft mode so no interference would be coming from there.
Anyone got any ideas?