general problems of the 1999 Oldsmobile Silhouette

When driving between 58 and 60 mph we get a vibration that can be felt in the steering wheel and the passanger floor board. We have had it into the dealer 4 times , they have rotated and reblanced the tires, but the vibration still exists, the dealer will not do anything about it. They say every thing is normall, but we took a ride in another 99 and there was no vibration. We feel that if you pay 25,000 for a car it should work perfectly.