Copy And Other Functions Not Stable

In my payroll program I have an option to make a workbook full of payslips.
Some of the times this goes Ok but more often it fails. I don’t understand why.

The program first creates a new workbook by copying the sheet PAYSLIP MODEL to a new workbook.

If an old version is open then this is first closed by the program.
The new workbook is saved as ‘Paybook January 2006.xls’ (in certain circumstances this goes wrong see below)

In this new workbook the sheet ("PAYSLIP MODEL (2)").is copied for each new payslip and data from the payroll is transferred to it.

The program switches between the two spreadsheets all the time in order to load the data for each payslip. (by using “Windows(theThisFileName).Activate” and “Windows(theFileName).Activate”.

Usually the first try goes Ok but repeated use gets into more and more problems. (the first time I manage to make 248 sheets in a workbook but the second time only some 49 and then it cant even make one anymore and Excel crashes. Only rebooting the computer gets rid of the problems. (Cant even delete the old files created because they are “In use”)

Below a list of commands and their error messages:

Windows(theThisFileName).Activate
Run-time error ‘-2147417848 (80010108)
Method ‘ _Default’ of object ‘Windows? Failed

Cells.Columns.AutoFit
Run-time error ‘-2147417848 (80010108)’ :
Method ‘AutoFit? Of object ‘Range? Failed