What this means: as far as I can tell, the CPU is running at 2/3 of its normal speed. For most purposes that's fairly irrelevant. More irritatingly, if I don't touch anything for about five minutes, the screen turns itself off.
Earlier today, every time I turned the machine off to go into BIOS or to reboot, it failed to turn itself back on again properly, giving a POST signal which means either graphics card failure or RAM failure. Clearly there's nothing spectacularly wrong with either of those, given that I had a month of uptime before today and it's working now.
I suppose that replacing the CMOS battery might be the next task.
(There's another, vaguely related, irritation: apparently if I boot the machine on battery power, when I plug it back in the system clock starts running twice as fast as it should.)