Revs rising & falling at idle - never happened before

[CharlesStevenson]

Our 1.6 manual X reg has, sometimes, started to surge or drop it's revs when at low speed.

We've owned it for around 7 months and it's never happened before.

Symptoms are:

1. revs rising & falling without pressure on the accelerator, and

2. revs dropping when pulling away in a low gear, as if I've slipped my foot off the accelerator or selected too high a gear when pulling away.

Our previous S40 1.8 had a noisy VVT (so I know the sound to listen for) but I can't hear any noise from this one so I don't know if that's causing this issue.

When this happened for my wife, and again today for me the electric fan stayed on for a few minutes once the car was parked up, but normally the fan never stays on when we've parked, even after a long motorway journey.

I've checked the water level & that appears correct, so I don't believe it's overheating that's causing the fan to stay on.

The only additional observation I can make is that the CEL is on, but that's been on for months & as it's a mid-phase car, when I plug a fault reader in there are no codes to be read.

I appreciate that the two might be related, but as this issue's never occurred until the last 2-3 days it seems perhaps unlikely.

Mileage is c. 145k.

I've searched on here but can't find any suggestions, other than " ... carb cleaner for the throttle body and ICV ..." so I'd appreciate any suggestions as to what might be causing this.

Thanks