How long has the system been running? If it has been several months or more, maybe the system needs a good cleaning. Heat may be building up to the point it causes a glitch.
If you are using the -advmethods and/or the -forcesse switches, stop the client and re-start without them. This will slow down completion times, but may help with stability.
I have 1 machine that has a similar problem, once in a while it will lose a wu. I scheduled an anti-virus scan daily on that machine to interupt the F@H client for a short while, letting temps drop down a few degrees. I know that machine has marginal cpu cooling too, so a heatsink upgrade would be the best way to go, just havent got around to it yet..... Maybe a heatsink upgrade is in your future too?!? The original Athlons were hot, hot, hot!
Check under Power Options and be sure the hard drive does not turn off, or go into suspend mode, also, I recommend not having a screen saver. Again under Power Options, set the monitor to turn off after 10-15 minutes instead.
good luck!