I have a similar problem with similar specs (e8500, 8gb ram, 560ti 1gb vram) and I too didn't notice it until using the high res texture pack dlc. It seems to go away with alt-tabbing for me as well.

I WAS able to recreate it without the texture pack but it took alot longer and some nvidia inspector tweaks were on like ambient occlusion and 16x af and 8x msaa. Tried for 5 minutes loading new areas vanilla and couldnt get it to happen thought I wont rule out the possibility that it could because it wasn't that long.

It's really easy to create for me at winterhold college going from the dorms to outside about 5 or 6 times. My 60fps would eventually half after loading the area where it was 60 the last time it loaded. Alt-tab and its back to 60. Don't know if the driver (295.73) is at fault or the game. I did have a similar situation in Batman Arkham City but I just had to alt-tab once and it only happened one time so I'm completely confused.

Let's not exaggerate now, LOL. 10-15fps would be a "slidershow".

High framerate really doesn't matter(since once it gets past a certain threshold you won't even see) and low framerate doesn't denote a slide show. Steep dips will however cause that and if you are starting from a lower fps to start that would make it worse. FPS is the biggest [censored] stat when it comes to gaming. The last rig I gamed on was terrible most games never saw over 30 fps however they all ran smooth because the frames weren't diving to 5 or 10 they were sticking pretty safely at 30. A solid 30 fps is not perfect but its far from a slideshow. Pretty much the same thing Half64 said.

I`m doing very well with this "slideshow"

actually, that 26 frames moment only last less than a second, just after entering new cell in a city( so after a load screen) and when doing a very quick spin. I would say it`s an almost zero cost.

the 30 fps is because I use fps limiter not to pointless use my resources. 60 fps or 30 fps are totally equal. but with 30 fps my v engine load only to 97-98 (not 99) % while temp is constantly below 80 on my msi hawk 560 ti card ( without fps limiter it ussualy went to 85 degrees). this lets room for those heavy cells when they are coming in.

bottom line: my gameplay is as smooth as it can be with my computer and I woudn`t change this setting even if I would eventually get myself an i5 2500k and OC it to 4ghz. (eventually I would raise some eye candy sliders but would still limit my frames)

then you sir have learned how to win at playing Skyrim

Sure guys - whatever.
My FPS never goes below 60.
I consider falling to 40 FPS (briefly) to be an enormous drop...
So if you're happy with 30 then what's the problem?
If it's dropping below 30 then I guess your settings are still too aggressive after all...

I have a simple solution. CLEAN install nvidia drivers. My computer works strangely and nvidia drivers need to be clean installed every time i reboot for steam games to run without stutter in outside areas. If i dont clean install i experience 1-4 second stutters every 10 seconds or so, but only in outside areas. Clean install fixes it every time.

ok, some people don`t understand: if I don`t use fps limiter, I have 55-60 fps, except when I enter new cell where I have for about half a second around 26-30 fps and then again 55-60 fps.

That's normal new cell loading try the remove ambient interior fog addon and see if you get better results.
Here is a link: http://skyrim.nexusmods.com/downloads/file.php?id=9571
I still stick with my point before 30 FPS steady with no screen tearing is far from a slideshow and not the worst. I like to have close to 60 since I paid for hardware that should be able to do that. Anything past that is [censored] and I normally leave Vsync On in most games. Sure I can get 120 fps in DX HR with everything jacked to the max but its no different than a solid 60. My last rig was only capable of holding 30 FPS tops in games but everything was always smooth. Take Fable 3 on PC for example(terrible btw) the game is tied to 30 fps and it runs with no chop or tearing. FPS is not as important as people think it is as long as you aren't taking dives. Take it from a person who outperformed 98% of players in top 50 US guilds for 5 years in WoW raiding with maybe 10 fps tops.