(IIRC playlist is a YC alumn)
I'll try with Firefox just to be sure.
I've "downgraded" to the 32-bit version of Flash 10 (the one installed by the Ubuntu package), it had the same results for both Firefox and Opera. The stuttering seems to happen mostly because my browser is busy or something. If I put it in a background tab, it stutters more. If I use my browser it stutters more.
Anyway, it seems to be a Flash problem, so I don't think it's anything you can solve :-(
Anyway, they appeared to have returned to the old brand now. Great site, though :)