Vertical Stripes on screen during gaming

Hi, I have N64 plugged in to Smart TV but using the white/yellow/red AV cable as my TV has these inputs at the back. The N64 turns on and shows normal welcome screen. Black background looks fine. As soon as games are played with white background, all I get is thick pink and green vertical lines on screen during all games. Virtually unplayable, can barely read text on screen. Have no other cartridges to try (I have only got Super 64 cartridge with heaps of games on), so don’t know if this issue is: compatibility/console/leads/cartridge at fault. Can anyone help please? n.b. I am technically limited so any replies will need to take this into account! Thanks….