Odd rev counter behaviour

[freezaburn]

GOLF GTI MK3 2.0L 8V '94  (2E Digifant)

When I depress my accelerator and bring the revs up to anything from 1200 - 1800 rpm and try and keep it steady anywhere between these values, the revs automatically come back down to idle! (Accelerator kept at same position).
Then, if I let the pedal go the revs take a dip down to about 100-200rpm and then come back up to idle at 1000rpm.
In some instances, the revs come down too low and she dies. If I do the same and bring the revs up to 2000rpm+ the revs remain steady at the chosen position - then if I let the pedal go th revs come down back to idle as expected. WTF!!? 

I have replaced all the following:
 HT coil
 ECU Relay #30
 Fuel pump
 TVB position sensor
 Temp sender (blue)
 Dizzy cap, rotar and HT leads
 Accelerator cable
 No ECU fault codes from VAG and she start and idles fine at a 1000rpm (CO2 is set at 28)
 (No crank or lambda sensors on this model)

Has anyone experienced anything similar to this sh!te? ANY help will be greatly appreciated! 

[geeksteve]

Cleaned the throttle body?

[freezaburn]

Yup, TB clean as a whistle inside.

[Screech16v]

Shouts vacuum/air leak to me 

[freezaburn]

Thanks - I think I'm just gonna replace all the vacuum pipes I can lay my hands on this weekend and let you know what the result is.

[Screech16v]

Also check ya servo 

[Wayne]

Shouts vacuum/air leak to me 

^^This sounds like an air leak to me.

[freezaburn]

OK, I replaced all the vacuum hoses I could lay my hands on as well as giving the thottle body another good clean.
The rev problem is much better, although not completely gone. The car also seems to be responding better now.

How do I go about checking the servo?

[Wayne]

How do I go about checking the servo?

Switch the engine off, pump the brake pedal hard, push / hold the brake pedal down and start the engine it should then sink slightly