... G'day Wallis,
my son has the 2014 14m Vegas and it's fine, then started doing weird things too until we re-tuned his bar. I suspect you have a problem with either of 2 things, either the 5th line is short and pulling the nose of the kite down or the front lines in general are longer than the steering, causing a slight over sheeting as the wind picks up.
Check the front lines from the Y point, making sure all 3 above the Y point are the same. Usually, the 5th line is short (or the fronts) are longer than the 5th line so put in an extension, trim it and make sure its perfect. Next is check the overall tuning of the bar making sure the steering aren't too short.
Give it a go and let us know how ya go!
Robbie