I observed similar behavior on 2 different seedboxes with rTorrent + rutorrent.
When i add 4-5 torrents with plenty of seeders - the download speed goes sky high and then:
- On one seedbox rtorrent just crashes and I had to restart it manually through SSH.
- On another seedbox it shows 100% on all downloads and then XMLRPC error and then the whole download process restarts from zero at much slower pace.

On first seedbox i was able to manually clean-rebuild + reinstall rtorrent and then it behavior greatly improved.

I tried transmission on both - and it heaves in much more stable and predictable fashion. The only thing against transmission is that it's web interface is very functionally limited (you can only add/remove torrents and see basic stats).

I'll be trying deluge sometime...

What size were the rams on the seedboxes?
If the ram is small and you add to many torrents at once then the ram can't write the files fast enough so it crashes. ruTorrent has a good interface but allways freezes, just don't load to many torrent at the same time.

Yes, i guess loading all at once was a culprit.