Hesitation under load and no CEL, could be a sign of gas starvation. It could be just a clogged fuel filter or a fuel pump going bad. But in order to diagnose a fuel pump, you need special equipment to measure fuel pressures.
Also O2 sensors have been known to produce hesitation under acceleration when they start to fail, although not triggering a CEL.

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"If everything seems under control, you're just not going fast enough"
-Mario Andretti.
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River City Bimmers Chapter
2005 X3 3.0i ZPP, Htd seats, Htd strng, Black Sapphire on Terracotta.
1995 ///M3 Coupe, Lux, MT, Htd seats, Cosmos Black on Black.