My room seems to stutter as I get near the last 3rd of a race, especially at high rpms. Why?

I've noticed this for the past few races, and I don't understand what's happening. My car seems to have less power and the rpms will "stutter" when getting near the red. My acceleration is much worse as well.

What am I missing? I am using manual shifting, I thought it might have been accidentally set to manual with clutch and I wasn't using the clutch, but that's not the case.