Strange idling issue

Hi,

I have this idling issue that began occuring and then becoming more frequent until it was happening once every 3 trips. It can be nerve racking when stopping as the rpm will be at 100-200 and almost stalls when normally it's at 500rpm when idling properly. Eventually, after 10 mins or so, it resolves itself. If i'm stationary there will be a big rev getting up to a few thousand rpm, and then slowly drops back down to 500 rpm and is fine.

I took it to a mechanic and they said they put in new spark plugs as the others were dirty, and that made it only occur once every 100+ trips, but it still occurs. They don't know what the issue is. I suspect it's becoming more frequent again. Today it happened and didn't resolve after 10 mins and still occurred after i left work. This time i noticed the oil was below L, so i put more oil in but it was still idling rough until i got the big rev when stationary in traffic 15 mins later, and then it was idling fine.

Anyone know why this could be? It's enough of an issue to make me want to sell. it.