Most likely its a heat issue. Download Hardware Monitor and check your temps. That laptop isnt built for heavy gaming.
Don't think that's the problem. I play games at 70 degrees (which I don't think it's a really worrying temperature) and this is a recent problem because I used to play these games smoothly and with no problems at all. For example, I bought Black Ops 2 in December, played it a lot with no problems until this happened. Thanks anyway..