Excel giving random #N/A errors

I'm running Excel 2007 on WinXP and have been running into a very frustrating problem every now and then.

Usually it's with a large-ish file (>4MB in xlsm) with many formulae (nested SUMPRODUCTs, SUMIFs, ec.).

When I'm working on it, it's fine. I save it, it's fine. But if I close it, and re-open, many of the cells are converted into #N/A errors. There's no pattern to which ones are converted -- they are in various sheets (never all in one single sheet, but also never in all sheets), and affect both cells containing formulae and values.

Even if I save in .xls format, the problem persists. What I've noticed is that once it starts occurring in a file, it will keep occurring in that file, no matter how many times I save (or save as with a different name).

Any ideas?

I guess I didn't explain very well.

There is nothing to trace back. The file has lots of formulae. One formula is:

• =SUM(R64:R72)

It works fine. Then I save the file, close it, and re-open it after a while. When I re-open the file, I find that the whole =SUM(R64:R72) formula has simply been replaced by

• =#N/A

If I re-type the =SUM(R64:R72) formula back in the cell, it works fine again. Until I save, close, and open it again -- there's no telling if upon re-opening all the formulae will still be okay, or if they'll be messed up again -- if they are messed up again, it's not usually the same cells/formulae.

It happens at random. I cannot post the file that it happened with right now since it has proprietary information, but I also cannot recreate it with another dummy file, because it is random -- I have no idea when it will happen with which file. I'll try to sanitize the file and post it later. But like I said, there seems to be nothing special about it. It's just a large file, and at random, simple formulae like SUM get converted to #N/A when I re-open the file.