Hi. I bought a FC Twin for someone for Christmas

Hi. I bought a FC Twin for someone for Christmas and we didn't get a chance to test it until now. Whenever we play any game, there are times when the screen shakes and gets double vision, making it impossible to play anything. We tried both NES and SNES games. With the NES games, the screen doesn't shake until we reach a dark colored land or scene. The SNES portion works sometimes, then will shake badly and the colors become a squiggly grey and green. Again, once this happens, the games are impossible to play. This doesn't happen with one game, but with multiple games. The games we've tried are Mario Paint, Super Mario World, and Star Tropics-- all games are brand new and never used. Is there any way we can fix it?