Why does CudaMiner slow down ?

[Speedy]

I run Cudaminer on a pc, then minimize the window and leave it. Shortly it will slow to 40khash or something.

When i go back and maximize it, it will jump to 160khash again.

Why does it slow down and how do i stop it from slowing down ?

(If i keep a teamviewer on the machine, the speed is fine. So i wonder if it has anything to do with the monitor being turned off)