Spreadsheet changing to Manual calculations and recommended fix doesn't work

[zookeepertx]

Hello!
Every now and then, completely randomly, Excel changes itself to Manual Calculations. There's no telling what workbook it will happen on or when. I've had it happen on a workbook that was operating fine until 2 minutes ago, then - POOF - it's now on Manual.

I've searched this site and I've googled and looked at everything I can find and there seem to be only 2 explanations out there:

1. The first workbook I opened that day was saved as Manual so everything opened after that opens as Manual too.
2. There's a macro that runs on opening either Excel or that workbook that resets calculations to Manual.

Unfortunately, neither of these is the case. I've checked all of the 3 workbooks that I open first thing in the morning and they're all saved as Automatic Calculation. And, the one that I open that comes in from another program - I just opened a new version if that and it's also set to Automatic.
There are no macros that run without me triggering them.

Anybody got any other ideas? It doesn't happen often enough that it affects my work; it's just one of those things makes me roll my eyes and go "Well, THAT'S just stupid!" Just annoying is all.

Thanks

Jenny