You mentioned that your tach drops quickly when your car misfires. When the tack jumps around quicly, this is usually an indication of an ignition problem. When the car is moving in gear the engine speed really doesn't vary that much. The tach, however, runs off the igniton coil and will jump all over if there is an ignition miss. I would check the coil primary connections, condenser if there is one, and all the plug wires as a first step.