Sticking revs

I've just got a new Mk2 Golf. 16v KR 1989. When I start it, it revs up to 2000rpm and then very slowly goes down to 1000rpm. When I then rev it, it will stick at that rpm and then if i rev it again it will stay at that rpm. I.e if i rev it to 2000rpm the revs don't drop, then if I were to rev it to 3000rpm it won't drop. The only way to get it to drop back down is to drive off. Also in-between gear changes the revs go up to 3000rpm and stay there. 

I can see that the cable isn't sticking when looking in the engine bay. I've checked for air leaks as well. I've also disconnected the ISV and the revs don't drop on idle. 

Just took off the intake boot before the throttle body and checked that both butterfly valves were closed, and then started it and the revs still went up higher than normal... I then disconnected the ISV wiring and the TB wiring and the revs dropped a little bit. I have also tried a working ISV off another Golf I have and no change.

I would clean all the throttle and ISv etc with carb cleaner.

I would then find someone to set the fuel/air and the idle.

You could try adjusting the idle yourself by turning the screw on the throttle however I would remember the amount of turns you do so you could always change it back.

The adjustment for fuel/air mix is in the metering head. It's a small Allen key head, can't remember what size maybe 2,3 or 4mm head (just remember its small). Ideally you need an exhaust probe to monitor the gasses before and after adjustment.

Timing would then be the final step.

I've cleaned the throttlebody and tried another known working ISV. I'm going to try another throttle body as I'm going to upgrade to a larger throttle body. The switches on this throttle body work fine so this will eliminate the possibility of faulty switches. I'm thinking it's more of an electrical fault due to the intermittency of the the problem?

Vaccum hose perished?

Get yourself a can of ez start anf just lightly spray the pipes it will be obvious if there is an issue,

I've had a look at the vacuum pipes and couldn't see any issues. Is the one that goes from the upper inlet manifold go to the brake servo?

Try ALL of the vacuum pipes. joins etc. use it sparingly and you should track it down.

If you just cant find it , give me a text on 07791001385 and I'll give you hand to sort it .

certainly sounds classic vacuum leak to me

Many thanks for all your input everyone. Solved the problem, the T piece connector that goes to the brake servo, two hoses had come off and perished. Cut them down and put them back on and now the problem is solved! The revs now drop much quicker and it idles much smoother, I've now adjusted the CO and idle and it is running sweet. Thanks