Your best bet is to edit your Xorg.conf file. Its found in
Open a terminal, su into root privilages, open your favorite text editor, i.e. vim, nano, and then add another line to your mode section that sets your monitor at 70hz. Don't know what to write? Use gtf. Just type this at the terminal, and it will kick back out a mode line you can paste into your xorg.conf
It should kick something like this
ModeLine "1024x768_70.00" *insert a bunch of numbers here* +HSync -VSync
See if that dosen't fix your problem.