My 2000 Honda Civic with 180k km on it started behaving weirdly about a month ago. After starting the car the engine would go up to 1.5k rpm in park, and sometimes would have a really hard time turning.

(the only way I can describe it is as though the engine is turning slowly for 10 seconds then kicking into gear).

I cleaned the IACV which didn't fix the issue, but 6 days ago we replaced it with a new one from Amazon.

Everything was fine for the last 6 days until yesterday where it had a rough start and went up to 1.5k rpm again in park, immediately after starting the engine.

Does anyone have thoughts on what it could be?

Thanks in advance!

Got new spark plugs in it

vacuum leak?