I figured it out! Loose cable on the monitor - can you believe that!? I wouldn't have figured it out had I not turned the monitor and made it much worse.
Your test was something I had also tries to help narrow it down, and you're right the snow wasn't on the image. ...but the moving snow really threw me off the monitor - it must be an artifact of the digital video input (sending across mutliple simultaneous frames of video).
Has the monitor seen a LOT of use between different machines? By the way which connector type is it using, DVI, vga, HDMI?