Sounds to me like your cpu is overheating under the strain. Have you overclocked it? I think you might be able to download a piece of software that will tell you how hot your cpu is running. As in all cases when downloading though, be careful if you do download some software; make sure it's from a legitimate website and be sure that the software you download doesn't try to install other 3rd party software programmes on your pc at the same time. Usually you get the choice to uncheck certain software bundled programmes so that they don't install on your pc. Be careful to spot those programmes; they usually show up at the start of the software installation process.
It could also be a dodgy stick or RAM. For that you could try removing a stick of RAM (assuming you have more than one in the motherboard) and seeing if it continues to freeze. If it does, put that stick of RAM back in the motherboard and remove the other RAM stick and see if it freezes then.
It may also be your hard drive that's playing up. There are free software programmes you can download that will run a diagnostics check on your hard drive and that will tell you if there's a problem with your hard drive. As always, be careful that what you download is a legitimate software from a reputable website. Diagnostic checking software for your hard drive can be a bit unreliable though in my experience. Your best bet would be to try a hard drive that you know works in your pc.
It might also be your graphics card failing (assuming you have a separate graphics card fitted into your mother board). If you do have a graphics card that can be removed, I suggest removing it, and trying one you know does work, and see if that solves your problem. That said, this doesn't really sound like it is a graphics card issue to me.
Unfortunately, I think random freezes and crashes are difficult to pin point because as you can see, it can be caused by several things. If you do not have spare parts (a spare hard drive, graphics card, stick of ram etc) that you can try in your computer, I think your best bet would be to take your computer (I'm assuming it's not a laptop) down to a computer repair shop and have them take a look at it. Shop around though, because some will charge for checking over your pc, but some do not.
Sorry this post isn't very helpful but it is my experience, freezing computers could be a symptom of a number of different things going wrong.