Windows 7 Home Premium 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.130828-1532)
System Manufacturer: Gigabyte Technology Co., Ltd.
System Model: H61M-USB3-B3
BIOS: Award Modular BIOS v6.00PG
Processor: Intel® Core™ i7-2600 CPU @ 3.40GHz (8 CPUs), ~3.7GHz
Memory: 4096MB RAM
Available OS Memory: 4080MB RAM
Page File: 4146MB used, 4010MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
Card name: NVIDIA GeForce GTX 560
Display Memory: 2761 MB
Dedicated Memory: 977 MB
Shared Memory: 1783 MB
Current Mode: 1920 x 1080 (32 bit) (60Hz)

That's all the important and semi-important info about my rig. Anyway, in Skyrim, I always get a little bit of lag when turning, fighting, whatever(because I have lots of mods) but it's always bearable for the first hour or two(the time can vary a lot). Then suddenly, it can go from a bit laggy to HORRIBLY laggy and I'm forced to shut down the game. For a few minutes after my computer is slow but then it speeds up again and when I restart the game a while later it's fine for another hour or two or whatever. Why is this happening? Is my GPU overheating or something?

EDIT: Dunno if this is important or not, but I never get CTDs.

Is my GPU overheating or something? You need to monitor this.

What framerates are you getting? Try FRAPS.

It can very well be a mod. What graphic/texture mods are you using? How many mods do you have?

Are you using vsync?

Maybe your 1GB VRAM is not enough for your mod's you have installed ????

I don't have a high resolution texture pack. I have an optimised one to replace the official high-res one but that is designed for 1GB cards. Also, this only happens after ages of playing and usually if you go over your limit you get crashes, not lag.

BTW I do have vsync on, ipresetinterval(or whatever it is) is set to 1 and in Nvidia Control Panel, it's set to adaptive.

I have tons of mods but few, not including the one designed for 1GB cards, are for graphics(besides an ebony retexture, distant decal fix, and other minor changes).