I asked my friend who happened to be over (who works at a local game xchange) about this, and he says that it could be a large number of things, but the most common is a faulty power supply. He says that when they come in, they're mostly the same type of PS but on occasion there are differences (he seems to think they're refurbs [which I wouldn't be surprised about.])

Also, Guitar Hero III is bad about locking up (particularly in online mode.)

Hit me up, GamerTag: Elition