Hey,
too bad your stuck with this problem: it just sound like your just unlucky with an unsually defect. (I never heard of such a thing, we have a '98 300, with pratically zero problems aside from a recalled alarm which wil soon be resolved. )If you noticed this and your obviously not lying, this sound like a serious, and DANGEROUS problem. If the dealer couldn't find anything, don't give up, i think you'd better find somone who specialises in this stuff, and test drive with them. Don't give up, and drive carfully!
(This may be of interest, there was some kind of officialy testin g in terms of quality of all makes of models, and the GS came to number FOUR! That's right, four not of the lexus brand, or certain models in a class, but overall all models in all makes.!)