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]