Increase in engine revs

[Srixon11]

Hi good afternoon, i have a a3 sline, 1.5 petrol, when pulling up and putting it in neutral or just idling the revs increase to 1200 for a second then settle back down to 800, I've been told it could be something on the vacuum system, a split or perished pipe, any ideas would be much appreciated 

[Khanb]

Mine does the same but I thought it was a limiter