hmm... I think I had that same shaking issue with every track where it was possible to have that 'shaking point'. Maybe I remember wrong, but I never liked those fastest cars because that shaking. I only did drive those which could not reach that shaking point after I met that issue.
What bothered me most, was that shaking started suddenly when I did get over some point of speed... not like shaking more bit by bit, like it would do in real. In GT4 it just started suddenly, and if you lifted gas littlebit, it stopped. I use only FFB10, so it was very annoying, cause my table could not stand still.
Edit: I also remember that it was possible to avoid that shaking by driving gentle from another side to another side of track... my wheel only shaked when it was in 'zero-point'. Anybody notice that way to get rid of shaking?