I have a '89 740 GL with 295,000 miles, and when I start it the engine shakes at idle. When I press on the gas pedal so as to hold the tach at 2000 rpm the engine smooths out, but about every 2 seconds the tach drops to about 1250 rpm and immediately jumps back to 2000 rpm. I have attempted a number of solutions. The distributor, the flywheel positiion sensor, and the spark plug wires are new. The engine temperature sensor was replaced about a year ago. I cleaned the throttle body, and it makes the clicking sound right when the throttle (butterfly) valve begins to open. I cleaned the fire trap. I installed a used ignition amplifier module (located on drivers side quarter panel next to the air cleaner) from a salvage yard with same bosch numbers. None of above remedies have helped. I am now installing a new timing belt as the old one has been on for 80,000 miles - too long! Before taking the old belt off, all three marks did line up ie the camshaft sprocket mark, auxillary shaft, and crankshaft pulley, so I am pretty sure the crankshaft pulley has not spun which probably would not matter. Any ideas would be greatly appreciated. Thanks!