I've got the original 2.66 dual mac pro and for the longest time the mac would randomly lock. It would be on for a couple of days and just lock, sometimes it would be a few weeks. I checked the console and did not see anything as to why, just that the logs stopped. I started by taking out drives (have 3 drives) which did nothing, then i started with the ram. I've got 2gigs of apple ram (4x512s) and 2x2gb of non apple ram. I first removed a gig of the apple ram (2x512) and noticed from multiple weeks it doubled the time i would see lock ups (could be a coincidence). I finally removed the last 2 sticks of apple ram and my machine hasn't locked up since. I haven't tried just the apple ram, so i'm wondering if this is a common thing? I've been buying ram from this company (forgot the name) for all of my macs and never had any issues. Is it better to not mix brands when it comes to memory. I also have a mac pro at work (2.8, penryn) also with apple ram and the 4gigs of ram from this vendor and no issues what so ever.