Marc,

Ok, it is not an Excel Macro and it is not the Auto_Open macros: therefore my first assumption was wrong, and we need to start from the very beginning.

I ASSUME (still assume, you do not provide the information) that you have the formula behind the particular cell, and this formula contains the custom function that you wrote yourself. If it is the case, then the cell probably is linked to some other cell(s). Try to add a LOG routine to see whether your function is called and how does it work changing values, etc.
When you have the log, then probably we could continue to investigate together.

>The macro run on cell calculation like any other formula. It's not at the opening of the workbook. Everything works fine but SOMETIMES the recalculation is not done by itself.