Welcome to LinuxQuestions.org. Check your /var/log/messages for possible clues of system crashes, specially about the xfree86. There's a bunch of log files there that you may like to look at as well.
Also, make sure that you have your onboard graphics card disable on the bios and that your ran xf86config when you boot linux to configure your new card properly (you will be asked for a couple of questions there, as mouse, keyboard, graphics card, monitor and etc... make they very precisely). That's all I can recommend