This is the wrong thread of DCC questions... it's for questions regarding connecting to networks. However, in answer to your question, if you are having issues RECEIVING files and it is, as you say, sporadic even from the same person, then it is almost guaranteed to be a problem on that person's side. Or, you are experiencing this only when RESUMING a download. If that's the case, it's a NAT issue in your router (search the forums for NAT or Resume and you should see more information).
