My 3.9 V8 Auto Disco revs to 1600 when driving then drops back to tickover, then revs to 1600 then drops back to tickover.

Sitting in neutral (or park) it revs cleanly to the red line but as soon as I'm driving 1600 revs and that's it.

Any ideas? It'd just had new plugs, leads (high quality stuff), genuine lucas dizzy cap and rotor arm. Could it be the ignition module, coil or ECU?

Sounds like fuel starvation. I'd check fuel lines, fuel filter, fuel pump and pickup in the tank ...

Just read you other posts.

It is very rarely the ECU.

Paul ^^^ has made some good suggestions. I would also check for air leaks on the inlet side and check the air flow meter.

I can get these ECUs tested and repaired if needed, but I seriously doubt that that is the problem.