Just thought I'd throw my solution out there. I had the shaking problem on a lot of courses like you guys described, so I thought it may be a framerate problem. I went to the Video menu in GT4 and turned the brightness down 2 notches (was at 2, now at 0). This elminated 90% of the shaking, and there is very little difference in the graphics. Hope this helps.
The only logical reason I can think of for that is because, a like on a PC.
The hardware has to work hard and when you turn the brightness down. The graphic's card doesn't have to work as hard. The graphic card won't have to create the same exact texture/graphic with a higher intensity (brightness) compared to a lower one.
Basically less load on the graphics card which in turn... help the graphic car to actually work more process, giving a "slightly" higher FPS which gives you a smoother feel of the game.
So...
Default settings: graphic process + output (brightness) = Load on the PS2 hardware
Changed brightness settings: graphic process + Less output (brightness) = Still load on the PS2 hardware but the PS2 hardware has relocated some of the process that did the output onto the graphic. In turn giving better response to the graphic portion which gives the audience a better FPS or smoother feel.
Hope that kind of made sense to you guys...
All in all, I would just wait for a newer copy of GT4 in the next few months or year(s).
Assuming PD will fix this problem in later versions.
If you find the jitters, I would suggest you use S-Video then and not RCA/components. Just do the normal and not the 480i; 1080i (HDTV mode).
I found out, I only got the jitters when I switch s-video to components for the higher rez.
S-video gives the whole game a smoother yet better then regular analog RCA (not component RCAs); but still obtaining jitter free gameplay.
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