My computer freezes sometimes?

It's happened 3 times today. I just made my computer yesterday evening and it ran fine. Today i raised the speed from 1066MHz to 1333MHz. Then after about 4 hours loads of horizontal lines and zig-zags came onto the screen and i turned it off. I thought it my be my monitor but i checked and it isn't. I also ran a stress test for 30 seconds and the temperature went up to 51 degrees. Could it be that my motherboard is getting too hot and i need a better fan?