hello I have a 96 Civic and and I have faced all these problems which you mentioned and Alhamdullilah have solved them with a little research, guidance of a few friends and ofc the Pakwheels community, Btw which engine are you running? cuz there are different factors for different engines, anyway hope I helped this is what you can do.
For your idling problem get your TP(Throttle Positioning) sensor checked and adjusted using a volt meter, also get your IACV(Idle Air Control valve) checked It may be clogged by dust or maybe damaged.
Secondly your car is not going above 5500 rpm anyway you try, and also you haven't changed the engine oil in a long time probably indicates a damaged crankshaft, please check and tell me if you have ever noticed your engine is producing a kr kr kr noise somewhere,