OS X V10.5 Leopard :: Files Sometimes Missing From Folders After Copying Between Macs?

We have a 2009 intel iMac here, running Leopard (10.5.8). We receive files from customers via browser-based FTP or email on this mac, and the boss puts these files in appropriately-named folders, and copies those folders into the appropriate place on our file server – a G5 (1.8Gz single processor 2.5Gb RAM) also running Leopard 10.5.8. All macs here are connected via ethernet using TCP/IP (of course) 

However, every now and then, there are files missing from these folders. For example one folder (that had 100 images or so) had 4 files missing after copying to the file server, even though the folder was dragged across wholesale, not the contents. All filename conventions are standardised before being sent to us, and do not use any illegal characters that may cause a problem between differing OSs or processor platforms. 

This issue may not be limited to these two macs, but there may not enough network traffic between other macs to show this issue on other machines – it only seems to occur when copying across large batches of files, and does not happen all the time. There is no one thing that sticks out that links the missing files – i.e. filenames/filetype, etc. It seems completely random. 

I thought that TCP/IP was supposed to check data packets to ensure that all data was sent properly, but no errors are received on either side. 

Is there some sort of maintenance (other than repairing permissions, which is done on a weekly basis on the fileserver) that I can run

Info:
G5 Power Mac 1.8 Ghz 2.5Gb RAM, Mac OS X (10.5.8)