Run flat tires?

My 13 year old DD, the 05 Impala had to go. The underside was rotted out. If I had a flat tire on the road, I would not be able to jack it up because the lift point was rusted out. So this leads me to the thread name. I bought a new car, Chevy Cruze. This thing came with the run flat tires. Well, we had a heck of a snow storm Sunday night. I was able to get car off road, but not in garage until I moved the snow. Monday night I go out to garage to go to work........The darn thing acted as if a tire was out of balance. It became very bad at 70+ miles an hour. Slowed down, came home and took the night off. I finally realized and came up with the idea of Run Flat tires getting a flat spot, or whatever is used inside froze to the tire knocking it out of balance. Drove the car around at slow speeds until the bouncing went away. I was able to get to 80 with no problem. Looked for it on the web, but of course no info. Has anyone else here had a similar problem with the run flat tires?