I don't know the specific problem, but mIRC's DCC resume is notorious for being awkward, so it's a pretty good possibility of being the cause. Anyway, DCC is designed for small file transfers, it's very inefficient and since mIRC/DCC isn't designed for large file transfers it can often lead to problems. You're better off using a proper file transfer protocol (ie. FTP) or a peer-to-peer protocol (ie. FastTrack, OpenNap, Gnutella, BitTorrent).
