Volvo: hesitation/missfire., volvo enthusiasts, vacuum hoses

Question
i have a volvo s80 2.4 170bhp, it has a very slight hesitation/misfire at low rpm in 2nd/3rd gear.  It was very bad, i have just done a full service including fuel filter and plugs, it seems a lot better but not right.  I know i need an oxygen sensor as the light has come on but the hesitation/misfire was there before it did.  I am a fully skilled mechanic with all diagnostic equipment needed, ive checked ecu for trouble codes, none stored. Any help or suggestions would be grately appreciated.  Thanks

Answer
i'm only an expert on 85 to 90 etc. i always check for loose vacuum hoses or cracked etc. and ignition wires is always high on the list. but it's only in one gear you said? so it might be a computer problem; you might want to check this site called www.brickboard.com there are very skilled volvo enthusiasts on there that know about known issues (they work at dealers etc.)