Dont know whats causing this
I think when you slow down and you downshift the engine disengages (cant spell) and the idle drop to normal (which is why the oil pressure does) and then it grabs the gear below while the car is still going slightly faster than the gear@500rpm which pushed the rpms up to the 750 range.
i think you are noticing it now that you car is lounder and you can feel the rpms more.


