And here I was thinking you only ran WoW and had these problems, lol!
LOL sorry I left it out, I didn't think something out of game would have an effect like that. It literally doesn't affect ANY PvE content, just battlegrounds. What could be the cause of this ?
A machine has many routines inside, things you don't see. In a sense each machine has it's own unique parts from several hardware manufacturers, each part made slightly different than the rest, but somehow always end up working together. A computer is like an abomination, several parts from different places but they work, maybe with slight differences sometimes, side effects if you will. :P
It's a bit different with laptops tough but still, not all parts are made by one single company and there is always room for manufacturing errors, ever so small but yet noticeable in very specific situations like this.
But I think this may simply be a driver thing, I mean, my friend formatted twice and the default drivers he always installed was the cause, he had to get latest from the net before installing those on the CD/factory reset, e.g. It's really hard to say why this happens, really.