Same thing happens to me in a worbook with no macros I have ever written. It only happens to me when I am dealing with large worksheet, like 10000+ rows. At the same time some rows get enlarged to almost the size of my entire viewable area. Very frustrating. First thing I do when I open large workbooks like this is click the first row, scroll down to the last and shift select then unhide. then switch to max zoom out and auto adjust row height for those that are monstrous. If I didn't have to do this, I would be slightly happier on the whole.