UKPassats

Hi there , got a strange one , 2010 2.0tdi bluemotion , got an odd one , now the summer is here , when I start engine 1st thing in the morning starts as normal and runs fine , later in the day when the air temp it a bit warmer say above 10/12 degrees upon start up it runs lumpy and hunts a bit , as soon as you've driven for a short while ( just as water temp guage starts to move ) the engine idles sweet as , performance wise nothings different even when it's a bit rough on start up , had the fuel pressure reg changed approx 12 months ago and the thermostat has been done 3 months ago , the fuel filter was changed at the same time as the regulator , during the winter it started fine no matter what time of the day as temps were colder during the day , just seems really odd and although not a major problem it's suggests something isn't happy when it's warm outside on startup , always run on premium fuel and oils changed every 12 months on the dot , and ideas ?
Thanks

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