Same here for one of my boards. If I reboot the PC and start cgminer once, this board runs at only 15.4GH/s @ int:53:2:850, but will run 100% stable for weeks at a time. If I restart cgminer something like 15 minutes after PC startup, it will run at 17GH/s (with the same settings), but only last 2 days 6 hours before spitting out LIBUSB errors. My other three boards each run at about 20.4GH/s at 54:2:850, also 100% stable for weeks on end.

I've also updated to cgminer 4.0.1 -- been running smoothly for 12 hours so far, though it didn't fix my stability issues at 950mV like I was hoping it might. So far I'm getting the same performance as I was getting with cgminer_drillbit 3.8.4 r2. You guys with stability issues with linux/rpi etc. might find it more useful, but with my Windows/x86 PC, there's hardly any difference.