Thank you very much for your answer. Indeed I started experiencing the segmentation fault with driver 6629, which is the one shipped with RHEL 3.0. That's why I decided to try newer drivers. Is your graphic board a Nvidia Quadro FX? Maybe this issue doesn't come up with any Nvidia board, but only with some of them. I wonder if the problem is connected with the fact that I tried Pymol 0.98 and 0.99beta, but never 0.97. I will let you know if I will finally manage to solve the problem.