Excel 2007 buttons suddenly stop working

I've never seen anything like it... I open an Excel spreadsheet, perform some basic formatting (bold, change column width, etc) and suddenly half of Excel's features stop working. The scroll bars don't work (but I can scroll with the mouse wheeel). Some buttons in the ribbon are disabled, including the Office button, but many keyboard shortcuts still work. If I close the application, it asks if I want to save changes, but the save icon in the QAT is disabled. Some spreadsheets work fine for a while, then one time I open it up and pow! stuff stops working.

It's hard to tell if anything in particular triggers it, since there are no error messages, just a subtle change of button color telling me features are disabled. I often don't notice it until I want to do something and I can't.

None of the other office apps do this, so I thought the installation of excel might be corrupted, and tried repairing it to no avail.

Anyone have any ideas?

Thanks!

If it helps:
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