It sounds like a case of warped rotors. Turning rotors is rarely effective for very long, simply because the rotors are thinner after being machined, and thus are now even more prone to warping than they were previously. Shops that try to save money for their customers by machining rotors, rather than replacing them, are actually costing their customers more money in the long run.
Step #1: Machine rotors
Step #2: 10k or 15k later, charge the customer to replace the rotors
Have the rotors replaced!
The next time that a shop suggests turning your rotors, ask for a price comparison between turning and replacing. You will see that replacing them the first time around is not much more expensive than machining, and is WAY less expensive than machining and then replacing them.
Also--NEVER allow a shop to tighten your lug nuts with an impact wrench. This is one of the leading causes of warped rotors. Demand that a torque wrench be used, and that they reference the correct torque for your particular make and model.