Lesse, I wonder what else you're having. I know tons of people will say "It's your processor" but according to my graphs and performance charts for those games, you should be fine in Mario Kart especially...
Now, if it does this the first time an area loads, it could be the shadercache. Try playing a game for a long time, and see what happens. See if the issues stop happening in a long play session, going between the same areas you've already been to.
Now, if it does this the first time an area loads, it could be the shadercache. Try playing a game for a long time, and see what happens. See if the issues stop happening in a long play session, going between the same areas you've already been to.