New Alienware 17 Computer Randomly Lagging When I Play Video Games

I got my Alienware 17 only a few months ago

Specs:

Intel Core i7 4700MQ @ 2.40GHz

3072MB NVIDIA GeForce GTX 770M (Dell) (and built in graphics from Intel that i don't use)

16.0GB Dual-Channel DDR3 @ 798MHz

A couple of weeks ago, I was running games like Skyrim and the Witcher 2 on my computer on ultra high graphics settings and with a lot of mods. Now, when i try to play them, they recommend low settings, and anything higher than that lags. I made sure that my Nvidia card was selected as the default card in my settings, yet it still lags. Unless I can find some help here, I am going to try to resintall my computer to factory defaults.

Thanks,

Will