Try running 4 WoWs on your main pc and see if they get lag spikes then. Might clue you in to where the problem is.

Perhaps run a traceroute to the blizzard servers as documented on Blizzards site at the same time as you are playing all 5 WoWs. If you see lag spikes along the way, safe to blame the internet at large.

Edit: Link to Blizzards traceroute page.

http://eu.blizzard.com/support/artic...rticleId=18888