Volkswagen: VW Polo (1.6l) erratic Revving, lambda sensor, obd ii

Question
Hi,
The car starts fine but idles erratically. With the accelerator pressed to the floor it will only get to 2000 revs before dropping immediately to about 1500 and then go back to 2000. It does the continually, as if I pumping the accelerator pedal. The only way I can get it above 2000 is by pumping the accelerator, once over 2000 revs it runs fine until i let the revs drop below 2000 and I'm back to the same problem. When i do take my foot off the accelerator, it almost stalls before idling erratically again.  

Any help would be greatly appreciated.


Answer
Christopher,
 Although it's hard to say, I would first try looking for any engine system vacuum leak.  Next might be an O2 (Lambda) sensor; I recently had to replace mine.  Otherwise, it is likely a bad sensor, especially if you have a "check engine" light, so you'd need an OBD-II scan to identify it efficiently.  
- Rick