2005 Cadillac XLR minor shake at 50 MPM

Tires and Wheels problem
2005 Cadillac XLR V8 Two Wheel Drive 4500 miles

I can't get a straight answer from my local guys. I replaced the factory Michelin run flat tires with BF Goodrich G Force (non run flat) tires. I keep getting a shimmy at 50 MPH. I took it to another shop who used a high speed balancer and filled the tires with Nitrogen. Little better but not great. The guys at Sams and Tire Kingom keep telling me I need to put Michelins on it. Another guys said I will need front brakes soon and the rotors is the cause of the shake. A 3rd guy tested the rotors and said they are fine. I don't want to spend $1,000 on a set of tires but is that what I need. To me, its either balancing (which they are), the tires are out of round (they don't think so) or they are the wrong tires. HELP ! Roger