There should be a cgminer command to list all devices, and I believe it tells you the bus/port numbers. On my testers the busses enumerate as 0/1 or 0/7 depending on machine, so I run two separate instances of cgminer, one pointed at bus 0, one at bus 1/7, for easier note-taking about which devices are on which hubs during burn-in. We run up to 7 sticks at 400MHz per bus (note the bus usage is equivalent to 5 sticks at 550MHz); any more and we had stability issues like you're seeing. We test on debian linux, which I guarantee will make a more efficient use of the bus than your Windows.