Flash stuttering issues

I have Win7 x64 installed on a Packard Bell (sub-brand of Acer) MH35 laptop, also known as Hera-C.

The computer works fine otherwise, but flash video has skippy audio, and occasionally locks up for a few seconds, only to then jump forward in the video when it gets unstuck. This gets worse if I upsize the video; if I fullscreen it it just gets awful. Conversely, the skipping diminishes a lot (though it doesn't entirely go away) if I switch to another window. I've noticed that when I'm on another window it tends to skip a lot more if I do things like scrolling the page. Thing is, it's not just video - Grooveshark stutters too, and that's audio-only. Google Music as well.

The laptop has an awful SiS Mirage 3 video chipset, but I don't think that's the problem because it happens even with the generic VGA driver that Windows loads when I uninstall the SiS driver. Rather, it seems CPU-related somehow.

The video chipset has made it a massive problem to run Linux on this computer (Ubuntu doesn't even want to start), but the two distributions that get the widescreen resolution right - PCLinuxOS and Puppy, the former via proprietary driver and the latter via generic VESA - both run Flash flawlessly (though they have other video-related problems that make them unusable on the laptop).

What can I do to fix this? I tried to disable hardware acceleration in Flash and it did a whole lot of nothing. I'd much rather get Win7 working properly than fight more than I already have in an attempt to get Linux to behave with that video chipset.

Anyone? The problem persists.