Help with my Mamba Max

Well I sold most of my stuff but kept a Mamba Max to use in my CRT .5. I'm using a Neu 1509 2D with it. When I select any power above 40% I get a weird high RPM stutter/cogging. Happens with my 10xl as well.

Also there is very little modulation in the first couple of steps for the throttle. The first step is very slow, then it takes a considerable amount of travel (on the Tx) before it gets to the next step. After that it seems fine.

Any ideas on whats causing these issues?

Thanks