Occasionally (maybe once/day) while I'm flipping through tabs of an Excel workbook the "normal" look of the tab is replaced with what looks like a screenshot of the tab. The font is fuzzier, rows are wider, and then I can't work in the workbook anymore because the "screenshot" is on top of what I am working on. If I save the file, exit Excel and re-open the file is fine. Any thoughts on how to prevent this?