Lines running up screen

The tv has been great but now that I have connected the computer we have lines that run up the screen while we watch tv. If we unplug the computer from the screen at night and in the morning turn on the screen and watch tv the lines are NOT there. But if we then use the computer and try to watch tv the lines are present again (whether we have unplugged the computer from the screen or not). However, ifwe watch a dvd there are NO lines even if the computer is plug into the screen or not.
Any thoughts would be appreciated.