Originally Posted by BillGrissom

I can see it happening because the shaky idle is a "comes and goes" thing. It often appears that the engine gets in a "positive feedback loop", aka a microphone too close to a speaker. The feedback might be from the shaking causing the IP to modulate fuel, which amplifies the shaking, at certain rpm's that match the "natural frequency" of the engine & mounts system. Being a "nonlinear effect", it sometimes takes a certain external excitement to get it started, and any little thing like a shaky air cleaner might be enough to push it over the threshold. Think of the "butterfly effect" in world weather. Sometimes a computer model even needs a little starting bump to get it to show oscillations.