My 81 Grey Market 280SL has lost it's power. I have checked the fuel pump, replaced the fuel pump relay and fuel filter. Plugs are new and nothing appears amiss. The problem: The throttle feels "mushy" and when opened rapidly the engine coughs through the air intake, stumbles, and tries to die. There isn't enough power to get out of the neighborhood. Looking for hints or access to a good (with pictures) trouble shooting guide. Any clues will be greatly appreciated.