It's the same every time, I have been to several indies to see if they can diagnose it; it goes up on the ramp and everybody has a good look, nobody can see anything wrong, and then it drives great for a day or two!
First place to start is balancing, the ST is notoriously difficult to get right and the wheels are prone for buckling.
Next up is the lower wishbone bushes, these were oil filled from the factory and crack with age and leak it all out. Mine were all done by 38K/6 years.
Next up is the more expensive one, the driveshafts. You'll be able to test for this by jacking the car up, removing the wheel and checking for rotational play in the shafts.