1998 Porsche Boxter

The car starts in the morning on my way to work. It starts when the mechanic looks at it, it starts when my girlfriend tries it. The majority of the time, it starts for me, but then it doesn't.

So, I replaced the starter, relay switch and battery. All the power comes on, nothing seems to be wrong. The mechanic said he replaced some wires, but any idea which ones it could be that he may have missed? Any other suggestions are greatly appreciated.