What's causing the steering wheel shimmy between 60 and 70 mph on my 99 Q45t with 84,000 original miles? Had the tires rotated and road force balanced twice and it doesn't seem to help!

My main thought at this point is the tires themselves - how old are they, how is the tread and what brand?
I say this because I have been down that road with a previous vehicle.

Do you know if your wheels were ever replaced during the recall by Infiniti? Not all were affected, but it's worth checking. If you're not the original owner, you can still get these records by visiting any Infiniti dealership in the country.

Have you done the tie rods? at 84k those are shot. The rims are old so they could be done too.

My brake rotors were not true. While I obviously felt it braking, it took a good mechanic to suggest it was also causing shakes at 60 mph. Makes sense. It is like having an out of balance flywheel on one front side. Turned the rotors and shake is gone.