trying opening up ping and doing ping -t htpc see if a connection issue occurring during the transfer?
And if that is the case (I think Kewlx25 is on to something here), check for newer firmware for WRT54GL. I remember my old WRT54G router having similar problems when I first got it. Copying large files over both the wired LAN and wireless would actually result in my connection completely dropping to the router. Sometimes I could simply wait a few minutes and it would come back, but in most cases I had to actually reboot the unit.