Your Acer could be experiencing heat problems. A slow buildup of temps would match what you're describing. When's the last time you pulled off the case and went over it with a can of compressed air?

Getting MTW and STW working on the newest systems is supposed to be a bit of a black art, one which I haven't tried. I hear the Nvidia drivers don't play nice with some older games, but I'll wait and see if some who has, you know, knowledge wants to chime in.