Anyone Problem copying files from Windows clients?

Let me premise this by saying everything was working fine on lion.

I have a mac mini server that i am using as a common file share between the many devices on my network. (Macs and PCs)

Since i upgraded to Mountain Lion the PC Computers (all windows 7) have serious problems writing to the shared folder.

It can read and copy files locally no problem
but if i wish to write, a large file, or a folder, the copy starts no problem (the login has write permissions) but midway through the copy, windows complains about not having permission to write to the folder and the remaining of the copy fails. (even though some of the files have been copied, if they are small enough)

I have tried to copy onto an external drive, which i gave full permissions to, and the internal drive inside, neither of them successfully copy files.

However, if use the same login credentials with OSX clients, it goes on without a hitch.

Any ideas on what the solution to this problem might be?