Hello!

What happens is that some surfaces don't seem to receive light from lightsources, depending on the direction I'm facing.
Here's a screenshot from what it looks like in the room with the mini-boss at Fort Snowhawk:
http://imageshack.us/photo/my-images/690/screenshot22vo.jpg/

I'm using a few mods, including one that affects lighting ("realistic lighting with customization"), so I disabled them all, including the official HD texture pack, but still get the same effect.
When I look around at different parts of the room, the dark parts are occasionally lighting up correctly, just to go dark again.
Whenever it happens, i.e. the surfaces go dark, my fps go down from solid 60 to ~35.
Not sure if the slowdown is related, or if it's just coincidence.
Also, the textures are still there, just very dark. In vanilla you can still see the surface texture, due to brighter ambient lighting, but it's still noticeably darker.

I've come across this bug a few times, but nowhere near as strong yet as in Fort Snowhawk near Morthal.

Bump - I notice it wuite often now as I'm playing a mage and use Magic Candle a lot, which makes the effect very visible.