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A loose or broken vacuum hose causes rough idling, and is the easiest repair of all. This is a very common problem since hoses wear out over time. Look in your engine compartment, and examine the hoses carefully for any signs of abrasion, breaking or loosening. Replace any damaged or broken sections of hose, available at most autoparts stores, and your rough idling will more than likely disappear.
Clogged filters are another common cause of rough idling. Inspect your air filter. If dirty, replace it. Also, a clogged fuel filter causes rough idling. It's hard to tell if a fuel filter is clogged or not, so a good rule of thumb is to change it at every general tune-up.
Bad spark plugs, bad sparkplug wires and a bad distributor cap are other common causes of rough idling. If it has been a while since you changed these components, do so. Most auto service centers can do this for you if you need technical expertise.
A timing belt jumping a tooth is also a cause of rough idling. This throws off cam timing, setting in motion a series of events that prevent your engine from achieving proper timing. This is a more in-depth repair and, if needed, should be referred to a qualified auto mechanic.
possibly ajusting itsself. any severe weather/pressure changes in your area? and if so does your cars cpu compensate for this (thinks its at a different elevation etc then resets its idle proper after a certian time? I have heard of this but not sure if VW uses it) perhaps ask the service tech.
indicates that the cam servo/solenoid is faulty and is not maintaining the correct valve timing for the rpm
if the valve timing is out from a skipped tooth it would be rough from the start , not run fine until it warms up