I get the "shakes" on my new slim NTSC-J PS2 with GT4. Mostly in the city courses with all the cars in the screen or at the rolling starts on complex modeled courses with all 6 cars in frame and moving. Seems to overwhelm the graphics chip just a touch.....
It comes in, shakes for 1-3secs and goes away. All the time the game is still playable. Not a big deal, but if they fixed it it wouldn't bother me either.