I’ve looked through various threads but can’t find anything that matches the symptoms being displayed by my 2002, RalliArt Magna.
It’s a manual with 178,000 and is unmodified (mine since new).
The car starts as it should, doesn’t suffer from high fuel consumption, BUT the idle speed typically sits higher than it should when you initially lift off, before settling down to 750. The delay can be 5-10 second. At times it settles to 1000, then steps briefly up to 1200, before dropping to 750.
e.g. Start the car, pull away and stop at a junction. Revs sit as 1200, then drop over a few seconds to 750. Pull away, drive along the road, lift off and then dip the clutch - revs could sit at 1000 or 1200 maybe it will behave itself – who knows!
There are no fault codes in the ECU, and there are no mechanical problems with the throttle mechanism or cable.
Current thinking is a faulty Idle Speed Control Value or an air leak. It also occurs to me that there could also be a problem with the oxygen sensor – given that the car is now 12 years old, does it have a limited life span?
Any suggestions would be gratefully received ...

Sounds like you need to clean out your throttle body

Don't quote me on this, but from what I understand you can check if it's the ISC by disconnecting it one the idle settles. If your problem persists then it's not the ISC, if it goes away it is the ISC.
Research this a bit more before doing it in case I am wrong, also I think you will need to reset your ECU later as an error code may get logged

Throttle body and butterfly. Had to clean mine a few times because of sticking.

Thanks for the feedback, I'll go back and look at the throttle body again.

Duplicate post.

And yes I believe you are correct and some O2 sensors can fail entirely or become erratic with age. I believe they can be tested. Try asking Madmagna if your first fixes dont work.