For some seeders (not all, but more than 1) will be unable to transfer data when other peers try to contribute to the piece
Do you mean that this happens to peers with some specific client softwares and not others? If so, which ones? Eg. vuze utorrent tranmission etc
It would help the developers if you post a level-5 peer log (from a peer properties window) for the peers that stop with this problem.
Go into Settings > Event Logging and turn up the default peer log level to highest detail, level-5, and maybe add more lines too.
Then after this problem happens, you can right-click the peer, go to Properties, and cut/paste the event log into here.
When the peer log is on level 5, it shows all the little protocol messages that the peers exchange, and then if something is wrong it can be figured out how to work around it.
I couldn't find this problem happening in my torrents, but I'm sure there are a few clients out there that act funny if they get block requests that don't follow a linear sequence.