Yech.
It sounds like your worksheet is corrupted (or just on the cusp of corruption).
If you do the same stuff on a brand new workbook/worksheet, does it work ok?
If yes, you may want to see if recreating that worksheet helps. (Do it in
stages. I'd hate do do all of the work and have it still not work!)
Just curious: Do you use lots of comments? I've really only had one corrupt
worksheet (well, that I can remember). It had tons of comments. When it
started having trouble, I blamed all the comments (and all the formatting
(inherited from a person who liked lots of different borders/fonts/colors!)).
After I recreated it, it's a lot more vanilla than rocky road!