well persistent hard freezes typically are some sort of hardware issue, software tends to cause crashes, temps cause straight out system shut downs... though if they bake "other" parts they can cause a hard freeze too... typically hard freezes are ram or psu related.
First lets be 100% certain it isn't the ram. Grab yourself memtest86, test the system ram one stick at a time. make sure to let it run a few hours each stick.
IF that doesn't turn up anything, i'd say try the temp thing. Pop the side of his case off and stick a room fan in the openning on full blast. see if the problem goes away.
Next you're gonna have to swap the psu out. If you have another handy that's great, if not, RMA it. if this doesn't fix it, there really is only one piece of equipment left that could be causing this.