Performance issues with multiple clients

Hey Guys,

when i'm using multiple clients for eve-online, i have "strange" performance problems..

First about my machine:
Intel Core I5 2500K
Gigabyte GA-Z68X
16 GB G.Skill DDR3-1333
Paalit GTX 295
80GB Intel SSD (Postville)
Windows 7 professional x64

My problem:
I can run 4-5 Clients at the same time, as long as not more than 2 of them are visible.
Performance for the 2 visible clients is fine (180 fps+ without vsync, usually using vsync.)
(would be able to handle more i think, but only 5 Accounts :0) )

But as SOON, as i maximize a third client - the complete pc gets very slow. Every Client
(also Windows, running Movie or playing music) starts stuttering with about 0.5 fps.
(Even the Mousecursor). As soon, as i manage to minimize ONE (random) Client, so that only
2 clients are visible again - everything is fine again.

This may be a hardware issue, but the performance of "everything" is normaly perfect.
Just the issue with 2+ clients, so it has to do something with the multiboxing.

Does anybody have any ideas about that?
Or some tests i can run to explore the problem?

edit: As soon, as the third client is maximized, CPU-Usage spikes, until minimized again :-(
http://download.dog-net.org/easyshar...0226012537.jpg

best regards,
dognose