This problem has come up before ...
I am not having any problems with OpenGL programming using RGBA and Double Buffering modes and the gears demo (compiled locally) is working great but the screen savers I have flicker severely. I turn off direct rendering and they are fine again.
I have a 16MB G400 and PIII450 with 440BX board, 256MB RAM. I am using the 2.4.0test7 kernel and the mga driver that comes with it.
If there is anything you want me to try test, please don't hesitate to contact me.
This was because the default visual offered by the driver was not double buffered. This is fixed in the current DRI cvs.