General :: USB Memory Stick Disappears Randomly?

I've built a computer based on a Gigabyte GA-H67M-UD2H-B3 motherboard that I intent to use as a home server. It runs headless with a minimal installation of Debian Wheezy on an 8GB Sandisk Cruizer Blade. Everything seems to be working well, most of the problems I had had been dealt with, most of the programs I wanted had been installed and configured, but there's one major problem I've no idea how to solve: on random intervals it seems that the system can no longer see the memory stick.

It could happen 30 minutes after a reboot or ten day later. It could happen while I'm connected using SSH to the server or away. It could happen when it's busy doing something or just idling. I could find no commonality. I've tried a different memory stick (also Cruizer Blade), a different USB port, a more stable OS (Debian Squeeze), none helped. The symptoms: everything already loaded in memory works. The webserver, for example, still accepts connections on port 80. However, nothing can be read from or written to the memory stick, so any webpage on the local server I try to access returns a 404 error. No new SSH connections can be made because sshd can't verify the credentials, existing connections remains active but I'm unable to anything useful with those, as no command can be found. No logs are written, naturally. Removing and reinserting the memory stick had no effect. The only thing I can to is to manually power cycle the computer.