head shaking?

[Peanot]

I put this in the veterinary section earlier but it may get more replies here.
Friend has a mare who headshakes, but not all the time. She has been to vets and they couldn`t find anything apart from that they suspected that the sun be the culprit as it is worse in the sunshine.
When she is jumping though, she doesn`t do it. probably because her mind is occupied?
Anyone any ideas how to help as she is considering selling her, but she has overcome a lot of other problems with her.
Thanks

 

[Super_Kat]

A face mask maybe?

 

[RunToEarth]

equilibrium net relief, fastens to noseband and cover muzzle.

 

[Parkranger]

when exactly does she head shake? In all gaits? Just canter? Schooling or hacking?