There are a lot of past posts on this subject. I've posted at least a half dozen. You've started in the right spot -- Fuel Pump Relay. However, I would suggest replacement with a NEW unit. I had an '84 190E that did exactly what you describe. Turns out the stock FPRs were not hermetically sealed and subject to internal corrosion. This led/leads to intermittent contacts. The NEW, replacement units are sealed to prevent this from happening.