Photoshop prefs corruption causes lines to appear

"We have had a couple instances where a line across the center of a document appears. It is a couple of pixels wide, and a negative of what is supposed to be there. It looks like you opened the document, selected a rectangular marquee the width of the image but only 3 or 4 pixels high, then selected Invert. The image looks this way on screen, and prints this way. The weird thing is that if you open the document on another Mac, it looks and prints fine. Trashing the Photoshop prefs on the Mac in question solves the problem. We have had this happen on more than one Mac. Adobe tech support concluded that the prefs must be getting corrupted (obviously) but did not have an answer as to why this is happening. We are using Adobe Photoshop 6.0 in a print publishing environment, and have been using over a hundred copies of Photoshop for the past 6 or 7 years, but we have not seen this until version 6.0 with Mac OS 9."