So I have 2 512MB chips in my Core Duo iMac, and ever since installing Leopard I have had some random freezing, and just strange behaviour. For example, I clicked to open an image, and my screen just went black. It has frozen randomly, sometimes the screen saver freezes, and It has also had the screen just dim so you could barely see it, and the mouse froze. Not to mention some weird screen artifacts. So anyway I just chalked it up to Leopard bugs, but my friend insisted It might be a ram problem, so I took out one of my 512MB chips to see what happened, and the computer ran fine for a week (no freezing, and no screen artifacts). So then I took the other other 512MB chip out, and it ran fine with the second chip in. I put them both back in together, and it started freezing, and having screen artifacts again. I have run Memtest overnight, and it finds no problems. So to sum it up, each of my 512MB sticks works fine on their own, but when I put them together all hell breaks loose I don't get it? Could there be something wrong with the Logic Board, perhaps something not working right with Dual Channel? Both chips are identical Hynix brand chips, one came with the iMac, and I got the other one on eBay. Sorry for the long post, but Im confused Help??