What do you mean one is fine?
When i had my 5800, in sportsman the motor spools up just as fast as a mod, but tops out at 24,000 rpm. That's just how it was.
When you're running the systems IN a car, you never feel the cut-out.
When you run them on a bench, sometimes they cut out smoothly.. othertimes it's like bouncing off a rev limiter in a car.
How does the other one cut out? Like completely? Have you checked the sensor wires? the connection going into the esc might be loose. Otherwise try the end-bell bearing. Is it smooth?