They are video files and appear to be coming (via NTL Broadband) from several sources. When they finally complete and verify they're fine.
It's just this strange simultaneous stop/restart which puzzles me.
Keeping the number down to 4 seems to help but increasing it to 5 or 6 seems to jam the brakes on again.
I can see that too many downloads may cause a freeze but this is different.
Weirdly, Shareaza can support upwards of 30-40 downloads with no apparent strain.