Just got an '04 WRX a few weeks back. The car has 53K miles on it and everything is stock. The car drives great in all city and highway conditions - except for one stretch of highway that always seems to induce a speed dependant vibration (70-80mph). The on-ramp is a particularly sharp right turn followed by the on-ramp. After entering the highway, the vibration slowly builds (over about a mile or two) until the entire front end of the car is shaking. If I take an off-ramp and pull over, everything comes back to normal and I can proceed like nothing happened - the car will drive smoothly. If i continue driving the vibration continues and eventually fades away and the car again drives smoothly.
Since I dont have any noise or vibration in the city at low speeds or when cornering, I assume it isn't the wheel bearings. I had the wheels balanced and rotated over the weekend and had an alignment done at the same time. The only other thing that comes to mind is a worn CV shaft that slowly spins off-center, causing the vibration. I haven't dug into the problem much and the alignment shop didn't mention any problems when I picked the car up.
Has anyone else had anything similar happen to thier car? If so, how did you fix it.