My thought is that Theou is on the right track with the background. I've experienced ghosting at mid-higher speeds as well but I think its an optical illusion. That is why its not showing up in your screenshots.
It mostly occurs when the background is solid and most of your background is solid and I notice it when I'm prototyping things in classic programmer art, a box on a gray background but it goes away when I add some background to it. I would guess that if you had more going on in your background than it mostly being the default gray you won't be seeing it.
If its not an illusion then perhaps maybe the monitor exacerbates that effect if its not of the best quality, I'm not sure. I know my monitor isn't the most high end but when I add an actual background it goes away. Try adding a more varied design to your background and see if it still happens. Anything will do, even scribbling a solid pattern and tiling it as a test.
Googled ghosting:
"When describing an
LCD or
flat-panel display,
ghosting is used to describe an artifact caused by a slow
response time. As the screen refreshes, the human eye still perceives the image previously displayed; causing a smearing or blurring visual effect."
I think your lack of background is making this more pronounced