Congratulations you have created a huge ground loop, what kind of extension cord? 2 prong? or 3 prong?
If it is the 2 conductor type you are eliminating one of your ground connections on the PC. Is any of the A/V equipment grounded?
If it were me I would eliminate all grounding at the A/V side. Cross my finger that was the problem. Cause chasing ground loops is a PITA.
Mark (sixoneeight) = 618