I used to have that problem alot on my other computer with C5; sometimes I get it on my notebook, but generally only when I go into the hair modeler.  I found that there are a combination of things that help.  When the screen locks, for me it's normally tied to processer usage.  I do a  Control Alt Delete and check which programs are up, then I shut down the ones not connected with system usage. That frees up some RAM to maneuver.  Another thing is to make sure all your windows are shut it the modeling rooms and texture room.  Those open windows sometimes make my videocard go freaky.