Why do shadows make my and Directional Light switch on and off?

Depending on my view of the geometry my directional light seems to be switching on an off? This only happens when shadows are enabled.. if i disable shadows then the models lighting is fine. Any ideas what the cause could be?

[Update] Have changed all shadow settings has no influence. I've Manouvered to a position in my model where the direction light switches off and disabled shadows which lights up the scene. It seems to be something with the shaders and transparency (i think).

I have a water shader in the scene if I disable that to a diffuse shader then the whole scene lights up again whereas switching it to a transparent/diffuse shader then out goes my directional light.. it doesn't affect a point light or spot light.

A transparent/cutout/diffuse doesnt affect the light??? [/Update]

Try changing shadow settings in quality settings.