aetherbulb

Hello, I am having a problem with my MB and the mechanic didn't find the solution on the first attempt. My car will occasionally crank but won't turn over. It will turn over if the car has been recently driven or has sat for a long period of time (6-8 hours or more), but it won't turn over when it sits for approximately 2 hours. It tries, but doesn't fire. If I try and fire it and then leave it sit for a while (like 30 mins), it will then turn over. Runs perfectly fine once started, and I have no error messages. I can get it started with starting fluid, but obviously this is hard on the engine and not a long term fix. The mechanic thought it was the fuel pump relay, but that didn't solve the problem. I was thinking it may be the fuel pump regulator. Any thoughts?