Ok, so you boot it up, the bong goes bong, the Apple gets a spinner, the screen goes blue, and then bam, desktop. Shortly thereafter, it freezes you say. Does it ever unfreeze?
I would be first inclined to say temps, if it is the early white slab iMacs then opening it up is actually reasonably easy. Let me see if I can find where I put the link to the instructions and I will post back.