I am having problems with the sound breaking up/sounding "wavery" on
"Youtube"

It seems to be downloading fast enough, ie the grey line stays well
ahead of the red.

(WinXP)

Anyone any ideas why & what to do about it?

Spend more money and get the faster service. You will get more water
through a pipe than a straw, and it sounds like you have an el cheapo straw
sized service. And then, according to where you live, that may be all that
is available, and it is time to put up that $$$$$K system that you've always
wanted.

Steve