When I start up Skyrim it runs flawlessly, and continues to do so for a while. But after about an hour or so, it starts stuttering or lagging pretty badly; when I look around it is very choppy. The only way to fix it is to restart Skyrim.

Is there anything I can do to stop this? I imagine the textures mods are causing the problem since it builds up in the memory, but shouldn't the 4gb patch stop this?

Not really because then your 4 gigs is being shared with the rest of the game. What are your system specs?

Error: Cannot provide answer due to insufficient information.

I shall concur with bump in the night, not enough info.

But I shall make an educated guess: you are running out of VRAM, which restarting skyrim would fix.

Lose those texture mods, I bet it doesn't happen.

GPU:GTX 580 (1.5 vram)
CPU: i7 2600K 3.4 GHZ
MBO: ASUS P8Z68-V
RAM: 8GB G.Skill Ripjaw
Windows 7 64bit

I'm using Realisitic Water, Vurts, Skyrim HD 1.3, Better Clothes, No Tint and Desaturation and a number of other small texture mods.

Much better info. You happen to be using either of the Nvidia beta drivers? I found with them I had this awful little problem where they'd at some almost random point decide to run the cards at P1 instead of P2 (effectively half the clockrates) and they'd stubbornly never go back to full speed, end result is half the FPS and yah heavy sluggishness all around. Resolution was to ensure the profile was set to 'prefer max power' instead of 'adaptive' for Skyrim. Before you try that though I'd suggest confirming this is what's going on by running some GPU monitoring tool and play the game until it starts happening and then checking if the clockrates of the GPU/shader/memory dropped at roughly the same time.

No, I'm still using the last WHQL drivers from October, I'd rather not bother with the beta drivers. I also have "prefer max performance" already on. (I just need to Alt+Tab to get out of Skyrim, and still keep it running, right?)

Oh well damn use the 290.53 beta then, believe me it was a night/day difference in Skyrim performance for sure and I've yet to meet a driver based crash from them playing this game or a few others. Pretty stable all around. Oh if you using something like Nvidia Inspector its got this neat charting tool that shows you graphs of various stats about your video cards and its a good way to kind of watch the progress over time. What you'd be looking for is a heavy and going forward drop in t he clockrate of any of the three monitored components, ie if the GPU clockrate goes from 772 to 405 and stays there that means the driver has turned it down to 'light gaming' mode and this is where yah that max performance setting will overide that and keep going at full rate.

If this does end up being the issue and resolution I emplore you to report back, this fixed this particular issue for myself and I've been advising others to try it when the situation sounds similar like this but I've yet to get a report back saying it was the cause and solution for anyone else.

Alright, I'll play some Skryim with the EVGA precision tool running and see what happens. I'll let you know what I find