Allright, I think I discovered something useful:

The increase in the upstream data sent when standing still and on /follow is about twice as high for drenai (female in my tests) as it is for humans.

When my human female warlocks had worked themselves up to 350 B/s, the drenai female mage was at 750 B/s.

That is the explanation why with my first team I had occasional disconnects with my mage - the only drenai in the team - and had massive problems with my 5x female drenai shammy team.

The character selected for posting on the WoW-US forum thread that is linked to in this thread here was also a female drenai.

Who else has this problem and can you confirm the drenai effect? =]