I fount the culprit with process monitor. It was the Steam Client Service, which I had deactivated some time ago. After restarting NS2 runs fine, game+plog show no freezes anymore.
Ok, I tried everything you suggested without improvements.
Process Explorer shows me no signs of a CPU stealer, NS2 has all the available resources for itself. Of course the CPU/GPU load is around 70-99% while playing. I even checked the…