Is it possible there is some kind of trigger for the freezing? Or does it seem completely random?
If you can find a trigger we can investigate this as a possible software issue;
If not it is likely a hardware/driver issue;
In case of the latter you could try some tests to try narrow down the problem:
I would start off by checking the hard drive with seatools:
http://www.seagate.com/support/downloads/item/seatools-win-master/
If it has no issues, you can check the memory with memtest:
http://www.memtest86.com/
I would start off by checking these components because they are relatively easy to fix (read: replace);