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My first thought is tires or brakes.
Check your tire pressure and even get them balanced again.
Brake rotors could have pad buildup on them. Either replace or get them turned. You can check this before hand if you have calipers and can measure the thickness of the rotor for variance. If you end up replacing the rotors, I suggest replacing the pads. Also be sure to lube all slider pins. Usually the buildup on rotors occurs due to lack of proper lubrication.
Otherwise check your suspension. Jack the car so the tires are off the ground. Grab the wheel at 12 and 6 O'clock and wiggle. Does it appear to have slop or feel firm? Slop = check your ball joints.
Do the same proceedure to the wheel at 9 and 3 O'clock. If you get slop here check your steering links (tie rod ends).
Keep in mind this is not a comprehensive check. There are other steps to validate worn suspension components.
Put an OBD reader on it... mine did that... and it turned out to be the number 7 spark plug LOOSE in the head... and while in there I replaced all the spark coils. Fixed it. Got the suggestion from this forum.. good job people. Prior to it being fixed I was just sure it was transmission problems.....
Sounds like tires, does this happen after you have been driving more than twenty minutes? My Pontiac Solstice has performance tires, not to be used below 45 degrees. If it sits more than a day or two the tires develop flat spots. Have to drive it 10-20 minutes to make the tires round again. Will not take it over 40 until it smooths out. When it is cold it feels like my wheels are squares! What brand and model of tire do you have?