911sc misfire 30ltr euro model

[dgp911]

Can anyone help with a very annoying misfire which happens when shifting gears. Car starts and runs fine when cold, idles well. After 20mins a misfire creeps in which only happens when I just lightly touch the throttle. With my foot hard down it runs fine. The warmer the car is or the warmer the weather is, the worse it gets. Not sure if this is a simple fuel or mixture problem or metering head problem.