The BT Vision box is probably about three/four years old and it's plugged into the electricity socket with a cable. The other channels seem okay, but the sport is problematic.
I have BT Vision and whilst I don't have Sky through BT, I do have a similar problem to you when I try and watch BT Sports channels - freezing, pixillating and even loss of sound.
For Sky, I use a Now TV box which is okay.