1988 Other Volkswagen Models Engine vibration

Engine Mechanical problem
1988 Other Volkswagen Models 4 cyl Front Wheel Drive

My 1988 VW Cabriolet idles quietly. When the rpm is increased to about 1000 rpm the engine vibrates violently until the rpm reaches about 1500 rpm. Above 1500 the engine vibrations quiet down and at 2000 rpm the engine is very smooth. The vibration occurs when the car is parked or moving.