Watch for another day and see if the problem persists or even worse. There're only 3 elements to successfully start the engine, Oxygen, gas and spark. O2 and spark is usually not the problem, it's usually the gas. If the problem persists, check for fuel filter and fuel pump. Sometimes, leak/condensation in the fuel line can crystallize and block the passage (especially when fuel filter is dirty).

Good Luck!

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01' 3.0i E53 (M54) with 164,000 miles
07' 3.0si E83 (N52) with 134,000 miles
08' 3.0si E70 (N52) with 91,000 miles