[V8] 1990 V8 loss of power at 1600rpm

[M h]

Odd one here. (Not me; the problem)*
Audi V8. 1990. It's a UK model, RHD, which affects a few of the instrument cluster diagnostics, but otherwise it's like any other 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 starts to oscillate 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, in fact.

So far, the throttle switch was a suspect, as water had got into it, leaving some verdigris around the critical pins 4 & 5, but this appears to be working.  I might put a new one on anyway, as a precaution.
I had been chasing a fault in the cooling system- the viscous clutch had failed on the cooling fan, and the car had begun overheating in slow traffic.  With that fixed, a test drive had gone ok. This new fault popped up a little later.

I'm making another diagnostics 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.