I think I may have seen the opposite!
A friend of mine had a very stable Mandrake system which I helped upgrading with a cheap Nvidia graphics card. Since then she gets occasional lockups of X.
After troubleshooting this (upgrading drivers, new installs, etc.) I have come to the conclusion that it seems like the X lockups occur if her computer is rebooted from Windows into Linux! It only locks sometimes and not directly after boot.
Could this be that the graphics cards driver is not cleanly removed in Windows? Or that the card isn't cleanly initialized in Linux?
Your problem could be the opposite. If you power off your system completely (yes the button the back of the chassis or pull the power plug) and leave it off e.g. for 10 seconds. Does your system boot nicely then?