black koetsu sounds weird

Koetsus don't sound thin....ever. So your choices are that the suspension has hardened, and it is time for a rebuild, or that your setup is not correct. Check VTA, lowering the back of the arm may help. Double check offset angle and tracking force as well and perhaps reduce the anti-skate a tad. You may also want to double check the setup on the Oracle while you are at it.

If some days it is sounding ok , it may have magnetic particle buildup fouling the movement of the cantilever . A friend with a steady hand can sometimes remove this with a piece of bluetac on a matchstick under a microscope .