i get this too on the A1200 too, although it seems not to the same extent that you do...
i think the crux of the issue is that one of the custom chips is overheating. usually related to pushing "double time" screen modes. just fit a heatsink to either alice or lisa - whichever one is hottest, or both, and the problem should pretty much go away
iirc, its the one that is at the front of the mainboard. right under the keyboard, so you may have to do some filing to get a heatsink to fit :-)
