On my 89 560SL, that specific problem was the engine temperature sensor. The fuel pump relay would prevent the car from running at all. The engine temperature sensor is by far less expensive than the distributor cap so I would start there. Be sure you replace the right one (there are three on my car). The one you want has two wires attached.

Do you have a manual or CD that will help you isolate the fault using voltage tests? If not, go ahead and replace that $40 sensor. It is a sma ll gamble.