Samsung TV Causing Pixelation

Had A Samsung TV in Bedroom, worked fine for ages then it started pixelating on some channels even though the TV said the signal strength was good. Did lots of checks on aerial plugged in another TV from another room in the bedroom and it was fine. Decided it was the TV bought another Samsung and it did the same so returned it. Bought a cheap Blaupunkt and it has been fine. A couple years have passed and I have been given a Samsung TV for the study. Plugged it in and now the Blaupunkt in the bedroom is pixelating on some channels. The Samsung doesn't even have to be plugged in to cause this just the coaxial plugged in. Any idea what is happening? The Panosonic in the lounge is fine.