How to stop monitor jitters?

[joe at actionline.com]

I just replaced my Sony 19" monitor (1280 wide 4x3 aspect ratio) with a
new Asus 24" (16x9 aspect ratio). Never had any problem with the Sony, and
the new Asus looks perfect on both computers that I use through a kvm
switch. But on one (only one) of my computers, the entire screen jiggles
for less than a second every 10 seconds or so.

What can I do to stop these annoying jitters?