I had the same problem some time ago, but now it's finished and I don't know really what solved the problem.
Do you have always the same problem ? when ? Is it when you start X ?
Have you tried "nv" driver ?
maybe you should post the /var/log/XFree86.0.log file so we can see.
Consider also upgrading to mdk 9.2, it's a very good distrib.