I recently upgraded to Mountain Lion direct from Snow Leopard. Ever since I have been getting weird vertical lines drwn on my iMac screen.
They don't appear all the time. They appear in windows when I move the window around the screen. They sometimes appear on the desktop behind the window - but not always. They appear in in Safari windows too. If I scroll up and down the line stay in the same releative position. That is, I can't lose them by scrolling them off screen. They are still there in the same shape and position when I scroll back down again.
They don't seem to affect Google Chrome. They plague Safari and Firefox. I have a demo version of iDefrag. The iDefrag data window has vertical lines at random where none should be present.
Here are some screen shots -
Moving a window about -
Sometimes the lines spill over onto the desktop behind -
About my iMac
iStat Pro Report
Safari scrolled up a bit - no change if scrolled off screen then back again
This happens irrespective of what User Account I am logged in to.