May be a daft question but what controls the engine idle speed? Does the ECU control this? Have noticed a few times now that it often ticks over around 1200 to 1300 revs? Then other times just under a 1000 revs. Is this a normal thing never had a modern VW like this so not sure!
Well make sure the timing is right first, then make sure you don't have any air leaks and then there is an idle screw on the throttle body,
Vacuum leaks linked to the brake servo are very common and can obviously be intermittent. See if you idle speed alters significantly when you press the brake pedal down
If you pump the brakes you will normally see a change in RPM but just applying the brake normally (once) should not significantly change the speed. If the idle is dropping when you pump the brakes, it suggests that your idle mixture is on the lean side (could be from a another leak somewhere or just set badly) Best way to rule out a brake system leak is just pull the hose at the 'T' connector and plug it up (on the T side) see if things have improved.
