After I was unable to locate a certain problem, I took the car to one of those "Diagnostic places" and paid them $25 to find the problem. It only took them 10 minutes to locate a vacuum leak hidden under a panel at the back of the engine. They saved me $25 worth of frustration back then, but I imagine that the charge more now. They just did diagnostic; they didn't do any actual repairs there.
They were so busy it was best to make an appointment.