Without knowing much I would think that you might have a fuel problem. Assuming that your spark plugs and other tune up items are in decent shape, I would check your fuel pressure.

1st I would take a fuel pressure test to determine a possible blockage or a bad pump itself. Considering this checks out.....

2nd I would to a leakdown test on the fuel line. Sometimes if you let your car sit (overnight or shorter) you can loose all of your fuel pressure through a small leak. If this happens then your car will take longer to pessure up to be able to start.

Again it is hard to answer this question with such little information. Maybe you can be more descriptive of when it starts poorly, doesnt start and actually starts well. Include time in your descriptions.

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1998 E320 150,000
1980 300SD 240,000
1965 190D 79,000