Hi there,
I am sorry to say that mine does pretty much the same. When I come back from a trip and start it after it stood for 3 weeks, it starts within 1/2 second. But the next morning it will take it's time. Within 15 minutes of parking, it will start immediately, but anything longer and I need to crank with or without pressing the accelerator.
My theory is also leaking injectors. With time, the leaked fuel evaporates and it will start easy.
One of these days I'll replace them all.
If somebody replaced the injectors, maybe he can chime in.
Good luck

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Reinhard Kreutzer
1987 420 SEL (126)