More often than not, it'll come back. It's happening to me too. It's just slow. Maybe the multi-threaded Finder threads are just busy doing something else . When they freeze, the drives spin but don't go clackety-clack.
Power dips on the supply.
Try one of the machines with a UPS, or in a different environment. These particular machines, since they're all from the same production run, might have a different power tolerance. Most screen savers patch the system, and the system doesn't like to be patched. It's an invitation to screw up the system. Darkside of the Mac is a really good application that doesn't patch the system.