I know yous aid it doesn't happen each time, but can you elaborate more? Does it happen at certain speeds? Have you checked your tires? Could be wearing uneven, which would lead to a possible alignment issue. Could one or more of your wheels be bent or out of round? Have you checked your tire pressure? Are you running run flats? Have you had your wheels/tires re-balanced? Have you had your suspension inspected? Just throwing some ideas out there.
