Macbook Pro 15" Early 2008 Screen flickering and distortion

OK so I'm currently using my sisters old macbook pro from early 2008,15 inch model, 2.5 GHZ Intel core duos and sometimes I run into problems such as screen freezing then flickering then screen distortion. Then I would have to reboot to make it work again. This happens quite often and is annoying. So I can't tell if its a graphics card issue or LCD issue or whatever. Any suggestions on trying to fix it? P.S. I don't really know much about Macs so yeah. Just pointing that out.