A year 2000, 1.8 16v XU7JP4.

When the car has warmed up and is idling, there is an intermittent miss.  I
call it a miss, but technically it probably isn't one.  When in the car, you
can feel the car gentle vibrate then stop, vibrate then stop.  Inside the
car, you can't hear an engine note change and the rev needle doesn't move.
You only feel it lightly 'shudder'.

If you are outside the car, you hear the engine and exhaust subtly labour,
then pick up again.

I have tried various plugs, have a new O2 sensor and air filter, have tried
various fuels, injection cleaners and have given the idle stepper motor and
throttle body a good clean.  I've no fault codes, it drives perfectly, never
stalls and always starts.

I thought my recent O2 sensor had fixed it, but this morning, it came back.
The only difference between today and yesterday is the fact that today it is
very wet.  I also wonder whether the idle stepper motor is being a little
sluggish, although it's not something I have heard of before.