Excel stopped calculating.

I have an excel file that is being used to record measurement results from an automated robot cell. There is no human interaction, but my file randomly stops calculating and running my macros. It might work for 400 parts or 4000 parts, it might stop after 10 minutes or it could run for 10 days, and there's no rhyme or reason that I can find as to why it stops auto calculating but if you press F9 (refresh calculations) it starts right back up and continues to work again (for who knows how long)

Does anyone have an idea why it will calculate and save hundreds of reports automatically with out a single hiccup then just stop working? The computer isn't even connected to the internet so updates shouldn't be involved and the randomness of how long it works 10min/10hr makes trouble shooting difficult

ThanksBilly Hughes