We had something similar happening, and it turned out to be a networker backup that was trying to backup certain files. When it was trying to back them up, it would lock them, and the error you’re getting would occur.
I believe the solution was to make sure that there is no virus scanning or backups of the Exec directories (though it might have been the entire Site directory). Hopefully some one else will know for sure.
I hope this helps.
Cheers,
Mike Parkinson