now that i think of it a guy at my track had the same problem when i got my motor, my friend un-plugged his sensor wire and it wasn't as bad. I knew he had a different speedo so i didn't worry about it.
I bet it's the MMP... There was a batch of them that had sensor issues. Something with the way the sensor port was soldered on the ESC's board. The group I race with went through 10 or so of them. We would take them out or the box, wire them up, they would cog and right back to Castle they went... Thats why we all run RX8s now.