Tire Bulge problems of the 2013 Buick Verano

We have had 4 tire failures with our continental tires. There have been 3 blowouts and a bulge in a sidewall. Two of the blowouts occurred at speeds less than 25mph over small potholes. Tires had 6000 miles and 100 miles on them for the low speed blowouts. No bent rims. All deemed as road hazard. Bulged tire covered by Buick dealer. I paid half for first blowout, all of the first low speed blowout, and continental covered the last because the tire only had 100 miles on it. First two tire troubles started when the car had 7200 miles on it. The next one had 6000 miles and the fourth had 100 miles on it. Contacted continental and they denied any problems with the sidewalls on this line of tires.