[LAU] usb soundcard: xruns and latency even with -rt kernel

I'm using an M-Audio Transit usb soundcard with the kernel 2.6.22-rt, as packaged by ubuntu. I'm running jack right now with -p 3 -n 512, and I still get xruns, typically of values less than .03ms, every few minutes. Increasing the jack latency reduces the rate of xruns, but I can't eliminate them without using settings large enough to cause noticeable delay. I've raised the priority of the IRQ-16 process (the one controlling the usb bus with the sound card); I also tried to use setpci to tweak pci latencies, but for some reason the settings wouldn't apply (when I read the values back, they haven't changed). I've posted the results of rt-setup-report.sh (plus lsusb) at http://artdent.homelinux.net/~jacob/rt-setup-report . Is there anything else I can try? Thanks, -- Jacob Lee artdent(a)gmail.com