I have that problem too, on my W2K machine, and had it on my W98 machine. I think going back to the older version of Flask fixed it on 98, but didn't seem ot help in W2K. I tried increasing the buffer, have not tried the codec playback settings. I am not too confident of that, though. But I will try that.
Even now, though, when I rip High Fidelity, even on W98, it is jumpy, just like you describe, it seems to play 24 frames, and skip 6 (at least that is what it looks like).
By the way, I run a Duron 600 at 900, and a Slot T-Bird 700 at 850, on Abit boards. Turning them down a couple notches didn't help. Heat is not an issue. Everything is totally stable. It may be hardware, but I doubt it.