Red Hat 5.0 -> 5.1 -> 5.2 -> 6.0 -> 6.1, then all hand-upgraded from there. nVidia drivers run fine
I bet you're all using KDE; try switching to GNOME. Some friends of mine in Mississippi were having the same problem, except their systems were hard freezing (no ssh/telnet in). Theirs was happening mainly during Q3. I set up a little .xinitrc just to run X and an Xterm, then we started running stuff from there. Never had a problem. I remember reading that some 3D cards & apps were having trouble with Enlightenment & KDE (that's actually why I first switched to WindowMaker under GNOME, then to Sawfish). Might be worth a try.