Seriously, ditch the Toyo Proxies. Junk, junk, junk. At both Lexus dealerships I've been at, we don't even stock them as a factory replacement tire. I'm not sure if thats the problem, but I would start there. Is there a way you could have the dealership swap some other tires on there....see if it goes away? Did the problem start when you got the new tires...or soon afterwards?