I had problems with the wheel alignment. Having a new set of tyres (of a different make) on the front seemed like it might have been some of the cause - the steering wheel went off centre and swapping them to the back made it go off-centre the other way....
http://www.teamlexus.co.uk/forums/viewthread.php?tid=1517
In the end I got a new set of tyres so that they all matched and lexus sorted out the wheel alignment to perfection. (I went to a tyre company earlier but they used different settings and my dealer said these were not the correct spec so be wary of going elsewhere too!) I suspected a faulty tyre or a slightly off-perfect, but still in spec range, wheel alignment caused my problem. Make sure the dealer gets the wheel alignment dead centre of the spec range!