I am having an issue with external loads causing the computer I have my Arduino connected to to momentarily drop out, which causes the Arduino to lose USB power momentarily and the port drops out and won't reconnect unless I physically remove and reconnect the USB cable. Since I need the USB connected in order to send and receive serial data to/from the Arduino, I was wondering if I could physically open the USB cable, cut the 5V line, and supply it via and external regulated 5V supply. I wouldn't cut the ground wire, but would tie it to the external 5V supply. I have read conflicting information on this (something about handshaking) but was wondering if that applied to Arduino's specifically.
If you are drawing more than 500mA from your PC then that is why it is happening.
I suggest, rather than butchering a cable, you get yourself a "wallwart" (Mobile phone charger) and use that to power the board. You really should work out how much current you are pulling before deciding on what PSU to use.
A problem with wall warts is earthing, you need to make sure that your PC and wall wart are connected to the same earth connection, so that they share a common ground.
If the current being drawn us only for a very short time then it might be possible to provide additional power using a capacitor in the power line. But plugging a charger in is much easier.