Texture changes brightness at random – why? (on iPhone)

Hello! For a couple of my textures on my game they intermittently change from normal brightness to a rather high brightness when playing the game on a device, swapping between the two levels of brightness at seemingly random intervals sometimes changing every second or two sometimes very rarely and sometimes not at all!

I have tried using diffuse as well as mobile/diffuse shaders both of which have the weird problem. The tiling is 1,1 and the offset 0,0 (very simple repeating pattern boring texture). The object also has a mesh renderer and mesh filter but that is it (no code or anything affecting it through code).

Any ideas on why this might be happening?

Thanks.