Graphics glitches following cursor on G5

Any ideas what could be causing this? I've been through at least three graphics cards on this mac, so I'm really hoping that's not the problem again. But, given that this glitch is showing up in screenshots, I'm guessing the problem is elsewhere.

This started immediately after I wiped my system hard drive and did a fresh install of 10.5 – which I was doing in order to hopefully fix some issues, not cause new ones!

I've done this on my system hard drive – upgraded to 10.5.8 and the problem persists. I then booted up off my external hard drive which is still at 10.5. So that means that the issue is not version-specific and not isolated to one OS install.

I should note that I'm not installing the OS in the normal way, not that I really think that would have anything to do with it. I lost my 10.5 disk years ago, so I'm using a trick to install the system from a DMG copy from one hard drive to another without rebooting. So, I installed 10.5 to my external drive, booted off that, and then wiped and installed it back to my system drive. Then updated from there to 10.5.8.

I suppose something could be wrong with my 10.5 disk image? I really don't want to go pay full retail price for a new copy just to find out that that wasn't the problem though.

It still happens in safe boot, so I guess that means it's hardware related?

It's not quite as bad as the picture indicates. Maybe it's getting better. I only get the trailing glitch behind the cursor when I scroll. When I'm just moving the cursor around nothing happens (except only some minor glitches when clicking on icons on occasion). And even when glitches do appear, they still clear off the screen quickly. So I can live with it. It's just strange, and makes me worry about future hardware problems.

There is one other anomaly too, that I'm guessing is related. Whenever I leave my computer alone for awhile (probably about long enough for the hard drives to spin down), when I move the mouse, both monitors flash a black frame before going back to my active desktop. I don't currently use a screensaver, so I may try that and see if it helps.

Strange stuff