Are you averaging zeros on the Garmin?

 

Thanks - that's most like the issue - stopped at shop for some tubes and set to "non-zero" avg.

 

No, excluding zeros also means excluding time, so you don't end up with a 3 min avg @ 1000.
You end up with 5 sec interval @ 1000.
And isn't Strava power just an estimate anyway?

 

If you have a power meter strava uses the data from that and doesn't estimate.

 

^this. It works fine now. Set it to exclude zeros. The Strava software must exclude zeros whether or not you tell the garmin to do so or not, ie garmin must capture all the data and pass to software. "exclude zeros" is just a display preference.