I have a problem affecting multiple users which work on PC's with four monitors on each. Some have Bloomberg add-ins and some do not. When they have multiple workbooks open and they alt+tab between them they often get either black cells throughout the visible sheet or see ghosting from the previous workbook they were working in. No other open applications seem to be affected and are usable. The problem usually can be corrected by either closing and reopening the workbook or sometimes alt-tabbing again. This has been going on for months and does not affect all users of Excel or the same workbooks. I should note that all PC's have been built from an image (users with issues and users without issues), Excel has been run without any add-ins and Office has been un- and re-installed. Any advice is appreciated.