It'd work fine up to the point I tried resuming a torrent, then after a few minutes it'd all go to pot.
A combination of switching from the official client to BitTornado (aka the Shadow's version) and going from plain ordinary ZoneAlarm 5 to ZAPro 4.5 (it seems ZAPro 5 has compatibility issues with a pile of programs but I wanted the ability to say which ports were blocked and which weren't) and all is fine.
Anyone else had this sort of thing? Is someone attacking the bittorrent 'network'?