I have a Toyota altezza rs200 automatic made in.

I have a toyota altezza rs200 automatic made in 1999 my altezza doesn't pick up any speed after reaching 120km/h I have recently imported it from japan after it reaches 120km/h it has a hard time picking up any speed with patients it may reach 140km/h at max upon check up the mechanic found oil in the spark plug chambers he sealed the gasket. But I still have hard time accelerating to higher speeds beyond 120km/hour