Fiesta idle revs fluctuating

[liamc20]

I currently own a 58 Plate/2009 model Ford Fiesta Zetec S Ti-VCT 120PS.

Over the last couple of weeks I have had a problem with my idle revs fluctuating up and down when I switch my car on, or when I am sat in neutral. It feels like something is chugging/struggling but cannot pinpoint what the issue is. Took it to my usual trusted garage last Thursday and the car was doing the same when I arrived - however this time the car completely cut out while sat with the engine on for 5 minutes or so. When they took the car into the garage about 15 minutes later, it wasn't fluctuating at the time (surprise surprise). They checked the ECU for error messages but nothing came up, and also did a full service changing the oil/filters and also the spark plugs. The service report came back clear as well.

The issue went away for 4 days but came back again last night. Just wanted to know if anyone had a clue what the issue might be or has experienced the same? I'm stuck with what to do now in terms of getting this fixed.

Any info will be greatly appreciated.