My 2002 had a shimmy last weeks during high speeds after buying new tires. Took it in and they found all the wheels were out of balanced. They checked boots, rods etc and said there were no loose parts and it looked fine outside of the balance. It did fine for a few days and now it is shaking worse during acceleration both at higher speeds and low speed. It also has moments of deceleration during hard acceleration. I have replaced the lines but not the plugs. Could this be a spark plug issue or something else?