GPS Accuracy Issues on Fenix 5

The GPS accuracy on my Fenix 5 appears to be quite poor, & nothing I change in the settings seems to improves it. I've changed the update rate to every second & switched on GPS+GLONASS, but the traces are still really zig-zaggy. I know GPS accuracy can be a little off around tall buildings, but it doesn't seem to be that great in the parks where there aren't any. If I look back at some of the activities I tracked half a year ago they don't seem anywhere near this bad. The GPS traces my girlfriend's new 645 Music produces look immaculate by comparison.

Is my watch faulty? ciq.forums.garmin.com/.../1350241.jpg ciq.forums.garmin.com/.../1350242.jpg