i have a very similar system to yours, i also had some trouble to get the right settings for me. Atm i run skyrim at 50-60 fps (i try to keep it at 60 because of micro stutt from sli) i also follow the STEP guide and i tested that by choosing the low resolution textures pack it gives me better performance with a little lost on visual quality. my settings are simple, choose high settings and set shadows distance at 8000, AA 2x, AF off, ugrids 7 and just for the long shots with bow, i set actor view distance to max.

my system:
i5-2500 @ 4.7
gtx 570 sli
8gb mem
ssd 120gb

I have a gtx580 too and 8 gig of ram (same type as yours), maybe your cpu is bottlenecking? When I first got my gtx580 I thought, right games you're dead. But they ran the exact same as with my 9800gtx+ lol, then I updated my cpu and zoom! I'm sure I read that Skyrim is cpu heavy or is that another game I'm thinking of?

If the occasional fps drops are only happening outside, the slowdown is probably being caused by loading new cells as you travel. Skyrim normally loads new terrain quite smoothly, but with all the texture mods you have installed, that is still quite a heavy load. Your bottleneck may be your hard drive, which simply can't keep up with all the textures it has to read.

I don't suppose getting an ssd is an option?

Edit: if this is the case, you should only get the slowdown every 15 seconds or so while running in a straight line, as you eventually cross into a new cell, triggering 3 or more cells to load in front of you.

Hi,

I have a SSD but installed Skyrim on my HDD @ 7200rpm because it was only 60GB, I have managed to free up 20GB so I am installing on this now.

I am now installing on my SSD and will let you know the result are.