calpate

I recently upgraded my stock brakes with the Brembo GT 4 piston Kit with one piece crossed drilled rotors and now I am having a "shimmying" problem in the steering wheel at about 60mph. I call it a shimmy because its not really a vibration and it doesnt occur during breaking. I have the American Racing 20" Shelby Razors with a 24mm offset. The shop had to add spacers to clear the calipers and now they are saying that the spacers are causing the shimmy. Their only suggestion is to order custom "hubcentric" spacers that are made specifically for my wheel. Is there such a thing?

Has anyone encountered a similar problem? And if so, what did you do to solve it?

Thanks in advance for your help.