How does the car run? Is it fairly smooth? If so, I would check for air filter restrictions. If the air filter is good, then check the catalytic converter . The reason I ask is because if it is ignition related, that usually causes an uneven firing, which will make stuff shake. If it is fuel related, it usually makes the car chug or cough. The only other thing would be perhaps a timing issue, but I think the computer would have figured something out by now and thrown a Service Engine Soon light.