that's my biggest problem. some service guys tell me this is how electric power steering works, it's numb on center, does only start to assist when a force is applied, which could be your own or some road imperfections, then sometimes it over assists and that's when you have to constantly recorrect it.
As I said before, the dealer checked tire pressure more than once. Did alignment, and I had 18" summers, 16" original summers, 16" winters(now installed) all of them same problem.
When I researched other lexus forums I found same problem a couple of times but with a lexus LS400, RX and LX (I forgot which models) but I remember I focused on people who had electric steering.
One of the guys said that on his car there a about 15 different programmed steering chips, and the dealer changed the chip to somewhat less sensitive which was not as quick as the original, but more balanced.
I did a printout, showed it to my dealer, but he said this guy had VGRS which I don't have on my lexus so it's a complete different storey plus he checked for any newer release of steering chips or programs for the IS250, didn't find anything. The only outcome he tested another car but didn't allow me to drive it and tried to convince me that it does the same. He didn't take it to the highway, and the straight road parts were very short. Needless to say he advice I go back to where I purchased my car and ask them for zero point calibration. Now I have to find time for maybe the 6th trip to my dealer. I just hope there is a fix in the end, or if this is the future of electric steering then we can kiss "fun to drive" goodbye.