Sensor Problem

I have a 2001 330XI 105000 miles. It wouldn't start and the dealer replaced a crank sensor to fix problem or so I thought. The car runs fine but every other week it will not start. It turns over when the key is turned but acts like it's not getting fuel or spark. The dealer says there are no fault codes and can't solve the problem. If left to sit for a few hours it starts up and runs for another week or two. Any idea's?

fuel pump intermitten? when the car doesnt start check pressure at the fuel rail, an easy way it removed the long skinny cover with 3 strips on it. removed the cap the looks like its for a tir valve stem, press down the valve and see if any fuel comes out. if it doesnt prime the pump by turing the key to 2 clicks but dotn trun the engine on. then check if fuel comes out again. no fuel, check wiring at fuel pump. if you the wiring is ok, is probally the pump.