I experienced similar problems with my Quadro-4, runny only two OpenGL apps.
It wasn't just dropping to unaccelerated, it was totally bogging down to the point that it took seconds to render even a wireframe box... even with a software implementation, OpenGL normally flies on my machine.
Unfortunately, I can no longer duplicate this problem. It was happening a few weeks ago, and the only solution I found was a reboot (restarting X was not sufficient), although if it happens again, I will try the console switching suggested.
It may be that I no longer have this probelm. The only changes I've made in the interim was deleting the "static" libGL.a and libGLU.a (hmmm?), and changing some nfs mounting parameters (definitely unrelated).
If I learn anything, I'll make sure to post.