Engine revs on emissions test

This is something I have come across a few times now, and it has been a volkswagen/Audi/skoda each time.

When carrying out a fast idle test, the revs will slowly move up and up to over 4k even with no movement on the throttle pedal. In the meantime the emission values are all over the place. It is almost impossible to get a completed test if you have the rpm pick up attached due to going over the rpm limit and the countdown resetting. Even without using a pickup the readings are high on co and lambda resulting in fails. The readings are ok on idle test.

I've only had one come back for a retest and that was from a vw/audi specialist who changed both oxygen sensors only for the car to behave in exactly the same way under the emissions test. He was none to pleased and said it was my equipment, so I had to get him sitting in the car trying to hold the revs steady before he would accept that there was still a fault present (he didn't come back again)

Just wondering if anyone else has experienced this and was able to cure it.