Process of elimination. Since you're saying there are no shake at time, comes and goes. My guess is that the pump is weaken or the seals in the rack is weaken too. Do you notice if it shake more when the car is cold or short driving time? In another words, it drives fine comes up more when the car has been driven over an hour or so? What I'm trying to get to is if the shake goes away when things are heated up, and all the seal expand and keeping the right amount of pressure; therefore it voided out the shake? Just my speculations, after all!