I think the moral of this yarn is never touch anything from PC World with a barge pole. They, like Curry's, are no more than a bunch of box shifters. My advice to anyone is find through word of mouth a small guy who will build you a custom computer to your needs and go with it. Most of the parts carry at least 1 year manufacturers warranty anyway.
OldBoy could be right and your unit is overheating. It's important to get cooling right and to give your case enough breathing space around it, especially at the back.
This link might be of some help to your son-in-law when he takes a look at it:
http://uk.answers.yahoo.com/question...5095356AAy9X2h