Engine won't hold 1600rpm

Odd one here. (Not me; the problem)*

Audi V8.
The car has developed an inability to hold 1600rpm. Which is quite irritating as this is close to gentle town driving. It Revs up to 1500+ Normally, but when it gets to 1600, it oscillates up and down by about 150rpm. Down, up, down, up, down, up (it's kind of repeatable)
It does this in park, neutral, or 1,2,3,D, and R. The effect in gear being like a loss of fuel supply.
Any slight increase in throttle pedal pressure simply results in a more aggressive loss of power.

Give it a boot from idle to above this, and all is fine. It revs through to the redline off load, and easily through to normal levels in Econ, Sport, and Manual modes.

It idles fine. Rock solid.

I'm making another cable to pull the codes (note to self- stop leaving cables behind when you move home), but in the interim, are there any similar events in other Motronic based systems?

*granted, it might be me! too.