Grounds- should either be everywhere if you are shielding for RF or single point if you are shielding for low frequency like less than 50 KHz.
The shells of the Video lines are either capacitor coupled or direct coupled to the earth ground of the power cord.
Check if the Audio-Video patch panels are grounded to the building steel, thru the mounting cabinet.
Replace the PC video source with an isolated video source and see if the lines disappear. PC's and video have not been a good mix in my experience.
Larry C
