Your xf86config file isn't configured correctly. Use xf86cfg -textmode to configure it. Make sure your monitor H-sync and V-hold are correct, your using the right driver, ati, and that your resolution isnt too high. Doing Xconfigs on systems like these is hard to learn and can be a pain in the ass but after awhile you get used to it. Also if your using FreeBSD you can try to upgrade using ports to the latest driver.