I haven't used serial cables in so long I forget how to check them. With USB if you plug in even a pass through cable (not device) it will show under Printers and devices in windows 7.

Since it is when running and you found electrical noise... Is this a conversion? Look at EFI harness. Is it anywhere near spark plug wires, coil or distributor cap? Or a arc from ignition somewhere? I think you found the issue. 40,000 volts near an EFI 5 volt signal does cause interrference.