My car would cut out without any prior indication that anything was wrong. Sometimes, I could restart it immediately as though nothing was wrong at all. On occasion, it would stumble a bit under heavy acceleration. I did find that my problem was accentuated by cooler moist conditions. In fact, I went over a month with no symptoms at all. First cold rain after that, I couldn't get her started again without the relay out and circuit jumpered. The diagnosis on the relay was VERY easy for my model (85 380SE). Pulled the relay and jumpered the correct pair of pins with a short piece of wire with the key in the on position and the pump started running as it should.
By the way, my pump has been noisy since I bought the car 6 years ago and it runs great. I do have a problem with the pressure leaking down when shut off, and I am hoping that the pump check valve is the cause for that problem and the noise. I'll let you know, as I have one on order and it should be here in a day or so.