Yeah mine happened after a full service, and they were supposed to have changed them through courtesy(VW UK involved) but when i had this problem my local tuner immediately knew it was either sparks or coils, when no faults came up they knew it was sparks, changed them and ran spot on!
I'd just go with the standard oem sparks, unless you are heavily modified which a colder plug is supposed to work better, check out the review section for the cooler plug, but i'd stick with oem spec, which i think are 7's.
Dom