[bluedog]

This car has a 3-cylinder-engine and as alwas on (older) Petrol engines, the idle can slightly vary, since it is not as fastly adjusted as on diesel engines.

If that doesn't vibrate, the engine is not on.

Do you have a Rev-counter? What does it indicate?

In advance:

There is two things at least, what you could do:

If the engine idles too low, you can say, that it is probable that the idle controller has a problem. That sometimes happens due to its age, and sometimes because some engine oil is sucked into the inlet through the valvetrain housings vent or the crankshafts vent and that clogs up the idle controller

If it is just vibrations, that may be because the engine mounts vibration dampers are aged or worn out.

As long as the engine stays on when it should be on in Idle, no of those two possibilities is really urgent to fix.

Since it is an automatic transmission, you should consider a change of the ATF regularly. I can't remember the interval for that. To change ATF regularly prolongs the lifetime of the transmisson and helps it to work as intended.

Have you checked the oil level of the engine? The other elementary thing about long term reliability of just about any car...