1997 Mazda Protege rough idle when engine is warm, poor fue

My car still runs great after putting a couple of repairs to it.
But there is one thing; engine starts perfectly fine in the beggining exept I noticed a lower rotations per minute (1000 rpms instead of 1300).
when I drive it and engine is warm, the engine idle decends at 500 quickly when I put my gear on neutral while the car is still rolling.
when I stop at a red light and apply the brakes, engine begins to rotate at lower 500-450 (the car would often rattle and sometimes stall when it rattles too much.
when you let go of the brakes while engine is warm and on neutral "while im parked'', rotations per minute would go up 1000.
I am also experiencing carbon buildup in the engine as well as on the muffler pipe and poor fuel economy.
I changed the spark wires and the car is preforming a little better but still same problem.
I did not open the idle control motor nor I have not spoted any holes in the air hoses.
for the past months I keep losing a little bit of oil in the engines before oil change.
i just want to know wt the problem might be.
real answers, real diagnostics
thank you