Ground loop.
How are you wiring up the AC side of the dedicated PSU? A pic speaks a thousand words here.
A lot of people connect AC Ground to DC -VE (or DC Gnd - in this case same thing). If DC ground on the PC is a fixed voltage different from the DC ground on the PSU, you get current flowing. This can either cause the USB to shut down, or in some cases cause the USB cable to get hot, as the current flows from one end of the USB cable to the other "trying" to make the voltages the same.
How are you wiring up the AC side of the dedicated PSU? A pic speaks a thousand words here.
A lot of people connect AC Ground to DC -VE (or DC Gnd - in this case same thing). If DC ground on the PC is a fixed voltage different from the DC ground on the PSU, you get current flowing. This can either cause the USB to shut down, or in some cases cause the USB cable to get hot, as the current flows from one end of the USB cable to the other "trying" to make the voltages the same.