Custom Function calculation problem

It really is hard to provide a specific reason for your problem without the code.

Can I assume that by UDF you mean a Function procedure that is being called from a spreadsheet cell?

contains plenty of references to worksheet ranges.

One suggestion: Are these references passed to the function through the argument list, or are they hard coded into the function? If the function needs information from the spreadsheet, it is usually best to pass that information through the argument list to avoid problems when the calling sheet/workbook is not the active workbook. Passing information through the argument list also eliminates (most of the time anyway) the need for application.volatile as Excel can readily figure out how the UDF fits into the calculation dependency tree.

Most of the arguments are for various discount percentages and each argument is set to the value of the worksheet cell with the same name range.

Is the use of values from the worksheet what is causing the problem when another open worksheet is calculated? After all these years I am surprised if there is not an effective solution.