2000 Mercedes Benz C280 Car won't start

My car won't start SOMETIMES, but it is happening more often. I usually have to let about 15 minutes lapse and turn the engine over 10 times before it starts. Also it has started shutting off while idling at a long light or in traffic and then again I have to wait about 15 minutes while trying to turn it over ever couple minutes. So far, it eventually will start. It is starting to get dangerous though with it dying at intersections and then I have to get help pushing it out of traffic or sitting in traffic while I wait for it to start again. Please help me, I have had it to my mechanic many times, he has hooked up the computer and no luck at diagnosing the problem, he said until it happens at his shop he really can't help me without just replacing parts randomly.