Well, it is losing time. Serene Support has suggested a newer video driver, because mine is a bit out of date. I'll try it. Someone suggested syncing with a time service. Sure - I can do that, but that's not addressing the actual problem.
As someone else suggested, it could be the act of checking time that affects time itself. So, I sat at my desk watching my screensaver all day, and checking my watch every 10 seconds, for 8 hours. And, indeed, my watch sped up! According to my system clock, my watch was 7 minutes faster!
