My Yukon XL 4x4 with the 8.1 liter engine has started dying intermidently while driving. It happens with absolutely no warning. I have had it checked by computer three times and nothing shows up. It has 100,000 miles on it, and I replaced the fuel pump laast week, but it didn't help. When this car dies, it becomes virtually impossible to steer, and is extremely dangerous. Any help would be greatly appreciated.