I'm not sure what you mean by graunchyness. 90 kmh is about 55 mph, which is on the low end to accelerate very hard in 5th gear, but at 140 kmh, which is about 86 mph, the engine is in its power range and is pulling smoothly. What happens if you downshift it and run it up in that range?
Welcome to the forum too.
Welcome to the forum too.