2001 Buick Lesabre vibration

After 2 years and 2,000 dollars worth of repairs buick dealer cannot find vibration cause, after 30 minutes on highway, a wave of vibration passes through entire car, and goes away for a few seconds, then it comes back again, when car is cold it will go 90 miles per hour with no vibration for 30 minutes, rare problem, says buick. We have 4 new mich tires, tire rod ends and cv joints, and one front motor mount, I have to check records for other repairs, thanks john