My daughter has had a 2003 Polo since February and it was running well until we took it to the main Vw franchise in Slough to have a full service. We then started to have a problem with the throttle resetting as if you had taken your foot off the accelerator pedal. This would happen in any gear and any time. Some days it didn't happen and others it happened lots. It would clear if you let the pedal off and pushed it down again as you were driving along. No ECU lights came on when it happened. No other problems with tick-over, etc.
Anyway after trawling the internet and changing pedal and coils, I saw one obscure post about a similar problem on another make suggesting that a brake pedal switch could cause a problem. So I bought a new one and fitted it and the problem has disappeared.
Now the interesting thing is that the switch that was on there is not vw but is exactly the same except it appears to have been modified. The white tip of the plunger is 3 or 4 times longer than the new one. This appears to have the affect and of cutting off the accelerator as if you were actually braking.
I find it extremely suspicious that it occurred after the service at the main dealer but can't prove anything. They have said it must have just gone faulty.
Hopefully this thread maybe useful to others. Please let me know if similar experiences had with main dealer. Or if further information needed.