Punto misfire on idle

Hi all,
My Mk 2 Punto 1.2 16v HLX V reg -petrol - has started to misfire on warm idle - revs erratic dipping between 500 and 700 rpm . No problems on cold
idle ( revs are higher) , or when travelling - no power loss or lumpiness when accelerating or at constant speeds.
More annoying than anything else - but I presume that neat petrol is passing through to the cat when it misfires - which probably isn't doing it much good.
Any ideas?
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