Track activity GPS is all over the place

I've been having some very weird results with my Instinct 2X and the track run activity, this is from yesterday's run:

(All screenshots are from Strava, but the map is identical on Garmin Connect).

This was last week:

In both cases the instant pace (during the exercise) and overall distance (after saving) seem correct, but the map is wrong!

I've been using the same watch for normal (non-track, mainly trail) running and the GPS is spot on, so I don't think it's a problem with the watch itself. 

I did have a go at setting the GPS to the max accuracy setting, with the following result:

Needless to say, this isn't an improvement :)  (And in this case, the pace was completely incorrect). 

I've also noticed there seems to be an "off by one" error when setting the GPS accuracy in the track run settings - select an option on one page (eg select "All Systems Balanced Performance") and on the current setting page it'll show "GPS Only". 

In all cases I've been running 95% of the time in lane 1, and have it set up that way.