550 High Speed Vibration

I have a 2000 550 Maranello with the modular (Barchetta) wheels and FHP. I've noticed the car has an odd vibration between about 90mph and 100mph. Above and below those speeds, it's smooth as glass (this is in sixth gear). I've also recently noticed a very slight vibration in the steering wheel at idle when the car is warm. I know the high speed vibration may be due to a wheel balance issue but I seem to remember reading (maybe on this site) about a know vibration with these cars at high speeds. Both vibration issues seem to have started or become worse since the installation of the 575M motor mounts. I can't/don't spend a lot of time at these speeds, unfortunately, so I haven't tried shifting down to 5th to see if the vibration is engine speek related or doing a lot of other experimentation -- I doubt that it is as the engine is perfectly smooth throughout the rev range at lower speeds. Any thoughts?

Thanks.