1999 Q45t vibrations HELP NEEDED

Me and the Infinity dealer in the area are having the hardest time pinpointing the problem of the vibrations in the steering wheel and the body of my 1999 Q45t at 60mph and above. It has the original 17" wheels and I have fitted them with the Michellin Sport AS. I had the tires forced balanced and also trued. The problem still exists......CAN ANYONE PLEASE HELP. I am so tired of not knowing the answer. The dealer has even shimmed the drive shaft to see if that was the problem, but the vibrations just moved in speed. ANY ADVICE OR INPUT WILL BE HELPFUL. Thanks in advance........