Screen shaking viberating?

I have a computer that has screen shaking or viberating.
It uses Win 7 and has a GA-MA78GM-US2H motherboard. It has
a AMD phenon II X3 processor.

Win 7 is not activated as of yet.

How do I trouble shoot this problem? The problem does not occur
every time I boot the computer. When the problem occures I have to
reboot the computer several times to get rid of it. It might occur 3 out
of 5 reboots.

Thank you,