Thanks, that's a cool tool.
Using the -l flag I am able to gain additional insight in my problem. i caught it today, and it appears it is triggered when i use a composition window manager and the gpu memory usage reaches 99%. When switching off compositing, the memory consumption immediately drops and the frame rates recover. When they get up to 99% the system seems to handle the situation much better. it may get a bit slower, but nothing like in the compositing case. on the down side i had viewer crashes in the past without compositing.
So who do i talk to now with these findings? the driver people (nvidia)? the window manager people (xfce, kde, ...)? the viewer people (sl viewer, phoenix firestorm, ...)? the gaphics stack people (xorg, libs, kernel)?