Problems networking with a PC. Error -31

Hi,

On Mac OS 10.6.8 on a Mac Pro hooked up to a LAN via a router.

My son's got a PC and occasionally, I need to send him large files, usually around 700Mb but sometimes twice that.

I can access his computer through "Go -> Connect to server" and start sending over the file.

Everything looks fine, progress bar showing and indicating the transfer, hundreds of Mb seems to be sent over.

Then suddenly, it stops with error message -31 (can't recall the exact wording).

After that, it's impossible to send over even the smallest file unless we both reboot our machines.

I have no idea why this happens and would be grateful if someone could suggest a remedy...

Cheers,

Dan