Hi Integra folks

This problem in my Integra 92 GS 5 speed 161k.
I just did regular maintenance: oil change, air filter, and front brakes.
And everything looks fine it drive very strong for 161k but when the engine is working in neutral gear the rpm is okay, between 700 to 800 but not steady, it’s going up and down you can see it in the tachometer, and you can feel that the engine is not working nice and smooth.


What can be the reason?
Is it some vacuum hose (I check already, look fine)?
Is it some sensor? (The car just passed smog)
PCV valve?

Hope you can help me before I take it to a garage and spend my money
thanks tom