I've had the same problem several times only I noticed it at 40mph or less. The cause was always either tire pressure or tire wear. If you have made any panic stops where you've skidded either tire (more than likely the rear, skidding the front could get hairy...!) you may have a flat spot in the tread.
Unless you've run over a railroad tie or hit a moose recently, I'd check the tires first.
