As so many of the internal system components have been changed, and it has been soak tested away from your house with no problems, I'd start looking away from the PC from being the culprit.
Have you got the PC plugged into a surge protector? Or is there anything nearby that could be causing interference? Power Lines, etc..
Failing that, try using the PC with various internal components removed/disabled.
Disable onboard sound or remove sound card
Network adapter, TV card etc..
See if the problem doesn't occur with one of those fitted.