Arrgghhh revs still fluctuating

[VWnut]

My 1.8 gti cabrio still has fluctuating engine revs, between 1000rpm and 400 rmp when at idle. I don't understand why, the k-jet is quite simple, i have replaced the auxillary air valve but no joy. Could it be running to lean (a vacuum leak perhaps??)
Any help would be much appreciated as this is starting to really p!ss me off.
Cheers
Tom

[VWnut]

I forgot to mention that when the revs are fluctuating, if i pull off one of the vacuum pipes from the servo or mfa (ie letting in more air to the TB) the revs will pick up and it'll be fine???