check that all the blades are on your motor fan. If a blade or two breaks off the imbalance causes the motor to shake/vibrate.
I had the problem on my 1994 325i, blades started breaking becasue the housing on the radiator/fan came loose and came into contact with the blades. Car had a terrible vibration and once I changed it, the car felt very smooth. Cost of Blade from the BMW Dealership (in South Africa) $27, Had it fitted in myself which too about 30 minutes.