There are two or three main causes for this to happen.
1) Tires out of balance.
2) Tires out of round - tire(s) have a bulge in them
3) Drive shaft bent or CV axle trouble.
Solution:
1) take your car to a tire shop and ask them to balance the tires
2) While the tire is on the machine and he is spinning it, have a close look at the spinning tire to see that it is not wobbling on the machine.
3) A mechanic will be able to tell you about this problem.
My bet is on problem number (1)