We have this problem as well
X consumes 100% cpu.
I can recreate this at will with a particular app it is an OPENGL app called Tecplot.
Ive logged it with the app developers but the call that seems to
cause all the problems is sched_yield().
I log in via another machine do an strace -f -p PID on both
X and the app with the problem but it seems X is okay
and the app is in some loop with sched_yield()
Kill the app and X still takes 100% cpu
Kill X and login again all is fine.....
This is on both RH8 and RH9 with the latest nvidia drivers