Revving Up and Down

Hi all, I have a 1.8 gti cabriolet, 1991 and am having a problem with it. On start up it starts straight away, but after a minute or so the car starts revving up and down. It idles steady after about 5 mins no problems. It does not cut out at all but it a bit annoying revs racing up and down. Any help with be much appreciated and thanks again in advance.

Look at guides to kjet issues. sounds like 5th injector/cold start issues.  Vaccum leaks can lead to hunting and fluctuations.

https://www.clubgti.com/forums/index.php?threads/k-jet-rough-on-start-up-newbie-questions.255867/

https://vwgolfmk1.org.uk/forum/index.php?page=topicview&id=big-block-injection_2%2Fk-jet-made-easy_2