Strange computer problem (appears as broken vertical sync for a minute or two when booting)

Factual Questions
1

I’m reasonably experienced with computer hardware, but this issue has me :confused:

Lately, when I boot, the display is all messed up. Like the title says, it’s like the vertical sync is completely broken. I get rapidly scrolling vertical distortion while the BIOS prints out its messages. It often continues through the Windows 7 login screen.

The weird thing - it resolves within a few minutes. The distortion lessens throughout the boot sequence and is nearly always gone by the time I’m fully logged into Windows.

Moreover - if I let the computer idle and sleep, the distortion returns.

Like I said, it resolves. I have no problems after a few minutes. It’s not a quick resolve, it’s like the problem fades. The distortion is terrible on boot, bad after 1 minute, minimal at 2 minutes, gone at 3 minutes.

Interestingly, if I have let the computer sleep and then wake it up, I can solve the problem very quickly. The Windows desktop will be heavily distorted, but if I open one FireFox window the whole thing disappears.

I’m running Windows 7 with a Radeon HD 4870. CPU is overclocked but GPU isn’t.

Anyone have an idea for what’s going on?

2

Swap the monitor with one on another PC. Does the problem move with the monitor or stay with the PC? If it moves you have a bad monitor, if it stays with the PC it’s almost certainly the video card. Outside possibility is the monitor cable, you can test for that similarly.