My iMac freezes and crashes randomly every couple of hours

Hi, I've got an 2008 20" iMac it's running OS X 10.8.2. Every few hours it crashes. I have completely restored the OS X. I've put new ram in, it's now 4gb.

I mainly use firefox, word and excel. It usualy crashs when I click a link or button somethings its when I move the mouse. I usually only have one app running at a time.

It seems to be completely random to when and what will make it crash.

When it crashes:

sometimes the screens goes black, with the backlight still on

sometimes the screens goes white

alot of the time it has vertical lines running down it

sometimes it freezing but i can still move the mouse.

If anyone has any ideas as to what I can do I would be very grateful.