A couple of weeks ago, I installed an update and rebooted. The computer (Dual 2.5ghz G5 with 4GB of Apple RAM and (2) 250gb SATA drives) would not boot back up. It chimed properly but it never continued through the boot process. After much digging around and testing things, I figured out that if I removed the plastic heat shield and booted the system, everything worked fine. However, I could only replace that heat shield once the boot process had fully completed. Last week I started seeing the display go to black randomly and then come back after I hit a key or moved the mouse for a little while. Everything is disabled with regards to sleep and the processor performance is set to highest. The hard disks do not spin down, etc. So, I went to repair disk permissions and it said it couldn't find a catalog on that drive to repair. This was actually a positive thing, at least I thought it might be, because if this whole problem was the hard drive I could just replace that and be back and running. No go. I swapped out the hard disk for a brand new one and still got the same booting issue. I also tried replacing the plastic heat shield with one from another G5 we have here to see if the shield was the problem. It was not. One other thing I tried was running the Apple Service Diagnostic (internal program you boot to via CD) and it checked out all the hardware and came back with 0 errors. When the shield is in place, it doesn't sound like the drive is spinning up properly. Everything's way too quiet. I don't know what to do at this point. The machine is fine for my needs and I really don't want to purchase a brand new one right now (spent $6k on this one a few years back). I also don't want to go through having Apple replace the logic board for $1200+ or any other massively expensive procedure as the system just isn't worth that much these days. I'd be glad to do the repair myself if I could be assured that it would fix the issue. So far, I've been unable to nail down what's going on and of course my AppleCare just ran out a few months ago. Perfect timing as usual. Any thoughts or suggestions are very much appreciated.