Screen judder for some games

Some of my games work perfectly (eg Pacman) but others (eg Frogger) judder on screen when connected. I have bought replacements for the games that cause problems but they too judder on screen. I have replaced the coax cable but it has made no difference. The 2600 has been cleaned internally with pressure air and is completely dust free. Has anyone else experienced this and if so, how was it fixed? Thanks.