Has anyone experienced graphic problems with their PAL Super Nintendo systems before?

I have just got one and though most games work fine this one is having problems with a couple of games like street fighter and mario kart, though the games work they are full of graphic glitches - street fighter is fine apart from when your playing the game and mario kart plays fine but has problems when you choose which game to play and when you win the race and go to the winners screen

Its not the carts as i have multiple copies of street fighter and each one is the same

Any help or advice would be much appreciated