I had mysterious "freezes" with my Dual Opteron server with 2 MB of RAM when I got it a few weeks back. The first time I actually got a Machine Check Exception. I flashed the BIOS to a much newer version but it still froze (with no messages!). I ran Memtest86+ for a FULL day and nothing showed up faulty. HOWEVER, running the Bonnie disk benchmark would cause the system to halt! Sometimes it would halt immediately, sometimes it would last a few minutes. I'd get it to crash fasters if I ran a few at once. I thought maybe it was something to do with the SLES 8 AMD64 kernel and my motherboard (TYAN 2882) or the LSI U320 scsi controller. Luckily I had an IDENTICAL server which I shifted the disks into and that server worked perfectly even when I ran 4 bonnies simultaneously (one per HDD). So I started testing each RAM stick one by one in the faulty machine and sure enough it was one faulty RAM stick. The RAM was ECC so go figure...! I wasn't impressed that the system hung with no messages but I was glad to have found the problem. A pity I had picked the wrong server to set up first! So there we go. You can have a faulty RAM stick and memtest86+ might not show up any problem. bonnie -s 2048 (for a 2GB machine) however caused the error to manifest. Cheers, Mike