Could also be electrical...

...I've had mine sit for a couple of days and it did just that when starting, while other times it sat a month for a major repair, and started up smoothly as if it had been run a day ago.

Check the harness, and coil packs, as well as the resistor caps that sit on the plugs...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car