As a further, I have experienced a remarkable recovery. The iMac went to sleep with the stuttering XP running, and when I returned to wake things up 10 minutes later, XP was still running (of course) but the frame rate was fantastic, and there was not a stutter to be seen........ I have shut down XP and restarted it, upped the resolution, etc., and it seems OK so far. As for later, or tomorrow, who knows?