Tank Mounting problems of the 2006 Buick Rendezvous

In 02-06 I traded a '03 Buick Rendezvous for a '06 Rendezvous. Taking a financial loss on the trade, because, with a serious heart condition, I was willing to take the loss in exchange for the security and safety of a "new" car under warranty, signing a low lease" agreement. Instantly I noticed a problem with the rough drivability of the car. A right or left turn would result in the wheel pulling in the opposite direction of the turn while the "wheel" then "skipped" back around to center. The gas mileage was not as reported by gmac. I'm lucky if I get 250 miles per16 gal. Tank. The gauge drops 1/4 to 1/2 a tank in less than 10-12 miles, or registers 1/2 full after being "topped off". I notified gmac that I wanted out of my lease agreement due to their unsuccessful attempts to fix these problems. In 4 days gm couldn't balance 10 new tires or fix the gauge. I'm distressed and annoyed. Carl blacks sales rep, bob hascom, said he would initiate a "buy back" on 08-01-06. The matter however has been ignored. Waiting patiently 3 hrs. One day at their service depart. (after being instructed to come back after putting 500 miles on the car) for the above issues, I read a brochure on "wheel vibratiion", detailing "force or dynamic wheel balancing". Buick has put 10 new tires on my car over 4 days on the 2nd occasion. It stated that the inability to balance tires could be a "wheel to axle mounting error", which I suspect is the true problem. With the gauge issue, I never know if I am truly out of gas or have enough gas to get home. My sense of security is destroyed: the only reason for taking a $$ loss on the trade in the first place. Can this situation be investigated?.