Pocketscan P1490 and where is it?

It is always your suggestion about that problem, what really causing is one of your High tension wires get open, check them one by one on ohm meter, you may not feel the problem causing that shake when you start the engine and run at its idle speed at normal, but running it 6mph the firing power goes a little bit down making the miss fire or poor combustion of one of your cylinder causing that shake, wheel or bushing problem may caused shake, but surely you will feel your steering swiveling left and right, that was the sign of unbalance tire or suspension component problem.