M14x Freezes and locks up.

Out of nowhere, I believe it only happens when it's not plugged in, my alienware m14x laptop freezes up out of nowhere. It really becomes an issue when I try to transfer 70gb of data from an external hard drive onto here, an hour + long process, and it freezes in the middle of it, more than once. It's just really a pain because then I have to force turn off my computer, which can't be good for it. I just recently re-did this computer, reinstalled windows 7 and drivers and everything.

how do I fix this?

It has 6 gb of ram, with a nvidia Geforce GT 555m graphics card. "Optimal resolution" for this laptop is 1600x900, but I run it at 1366x768 because I read up on my graphics card and read that that is what it was made for. 

help!