So I get a good FPS in every town and every landscape but when I am exploring a dungeon or being anywhere other then outside my fps gets a lot seemingly random fps drops.
Since it is known that the amount of shadow detail is eating away performance I lowered it's setting but it does not seem to help out much. It is extremely annoying and I just can't seem to fix this. Anyone got any useful tips for me to use?

I fixed it by lowering shadow res. from 4096 to 2048 but keeping the other shadow related lines to ultra values.

If it doesn't work try lowering shadows to high or even medium in the launcher, it should work!

Sure...when posting for Help in a Hardware /Software forum...Give as much info on Both your hardware /Software.

Lol...Rubs crystal ball....Um i think your voodoo 2 is out of Ram..

Orange = Try to notice if it is happening soon after a load door in a dungeon...

it is because dungeon is full of dust/fog/light, that kind of things is handled by CPU, not GPU. If you're playing on desktop, try to overclock your CPU, it will helps, or wait for Bethesda to fix it (doubt it will happen)

P.S : Sorry for bad english

No need, you did just fine.

As mentioned, lower your shadow quality.

In the skyrim launcher, got to options, then advanced, ad select medium or low shadow quality. Run the game and see if that helps

I've had the same issue OP. Since updating to 1.3 dungeons are unplayable and no matter how low I set shadows it has zero positive effect on performance. Probably going to have to wait for 1.4 or new drivers and pray it fixes it.

Update on the situation

I have installed a piece of software that calculates my fps and to my surprise I have a steadfast FPS of 60-40

Thing is, I still get this weird lag that is just annoying.

It sounds like stutter.

Try the borderless window mod mate.

Hmmm, see if limiting your FPS helps any -- http://www.skyrimnexus.com/downloads/file.php?id=34

Basically what causes stutter like this is when your FPS fluctuates too much, especially between 60 and less than 60 as outside you probably don't fluctuate as much being capped to 30.

I have the same problem. I found out that the slowdown is caused by fog and other transparent particles in dungeons. Since the CPU is rendering this along with shadows, pathfinding & AI, it causes massive FPS drops. Change this setting below to a lower number and you'll see an improvement. Lowering this actually barely makes any difference visually. It just makes fog and particles less dense.

Change this setting in SkyrimPrefs.ini:

[Particles]
iMaxDesired=250

You can go lower than 250 if you want even better FPS.

Yeah, there are certain dungeon rooms that do this to me too. Such as those round staircase rooms in forts. Then I'll go to the next room and it will be gone. Fraps reads 42 fps but it feels like half that much.

It never happens outdoors or in towns, just certain dungeon areas...

I occasionally get nasty framerate drops on a 3.6 GHz C2D + GF GTX 560 Ti while indoors. Almost always in Dwemer ruins where multiple shadow casting lights can be seen, though some staircase tower top rooms have similar issues.

From the mouse lag, it feels like the GPU is overworked, badly. Maybe the crappy lighting technique's just forcing the game to render everything a dozen times to shade things properly.

Thanks - I get a similar effect to the OP, and will try this. Especially if it doesn't noticibly reduce visual quality. I hate the stuttering every time I go underground. Who knew in a huge, open world game with steady 30fps outdoors, seeing dust + candles in a tiny cave would cripple your PC! Overall I can forgive BGS for most mistakes in the game, but this one is ridiculous. Creeping around a dark, dank cavern is not when you want to get all kinds of game-breaking stutter.