This i had with another car of mine(vauxhall). I had balancing done, four wheel tracking, new disc's, track rod ends and still the problem occurred. Only this all happend when i had these new alloys fitted.
It turned out to be the spiggot rings that the wheel fits on to the hub, they were just a few mm out and on putting the wheels on they never sat centre enough hence all this wheel wobble and vibration through the seat. I changed the wheels back to Vauxhall ones again and it was perfect.