I have an 06 Cayman S that is lowered with aftermarket wheels and tires. When I drive over 90 MPH under full throttle, the car will shimmy left to right aggressively. I have owned other sports cars and never experienced such problems. If I gradually increase speed, the car is pretty stable up to 130 but under heavy acceleration after 90 the car will pull left to right aggressively and become very unstable. I am not sure if it is an alignment issue, wheel/tire issue, or even a drive train issue. Any thoughts?? Thanks!!