This so called "mechanic" is ripping you off, trying to learn how your car works. Take your car to a authorized BOSCH specialist and have them check it out. This is a typical mixture problem that you are having and as you very well said can have a million and one causes. It needs to be researched by an experienced Bosch injection specialist.
There is also a small possibility that your timing is off. I assume that it has been checked out already.