I've just installed the 23090 build and it is showing radically different network behaviour to the 21886 build I had before. In some respects it seems like the uTP is being excessively sensitive and keeps choking off my bandwidth, and turning off uTP made things a lot better. However, I think the culprit is just tcp_rate_control. Setting this to false seems to cure the problem. I don't know if the setting wasn't working in the 21886 build or whether something is broken in the 23090 build, but they are certainly different. Or maybe uTP bandwidth control wasn't working at all in the earlier build?