Experienced similar problem
on our
40 inch Sony LED-LCD TV and the culprit was the HDMI cable. Interestingly the HDMI cable was seemed ok on our Sony home theater system but further tests showed that the set up was sensitive to some turned on and off appliances and electrical devices.
We replaced it with the one that comes with one of our Sony home theater systems and the problem was resolved. Also, make sure that your PC is set at 1920 x 1080p resolution.