2008 Saturn Outlook Vibration

I am having problems with vibrations once I reach 55mph. The vibrations are worse at 55 and get a little better the closer to 75mph that I drive. I had a 4wheel alignment done about 2 months ago and then purchased 4 new tires. Within a few days of purchasing the new tires I started having problems with the vibration. This was not a problem with the old tires. I took the car back and had the balancing redone. Still vibrating. I took the car and had the alignment checked and the suspension checked. Both checked out okay. I am still having vibration. I have taken the car back a third time to have the balance checked again and asked that the tires be checked for being either out of round or having a knot. Everything checked out and they used a different machine to balance the tires that time. Again after a couple of days I noticed the vibrations again. What else can I have checked. I am very frustrated with the entire situation.