Content originally posted in LPCWare by Tsuneo on Wed Nov 14 06:14:52 MST 2012
If you would have a hardware bus analyzer, it could show better clue.
But if you don't have any, try this test.
Insert a USB hub with external power supply, between the PC and your device.
PC <==> self-powered hub <==> your device
In this setup, cold start the PC.
A self-powered hub supplies 5V to your device, while the PC is in power off.
If it is power supply problem of your board, this setup should stabilize it.
Tsuneo