I just looked @ rctech, it seem there is quite a few people who run into such strange behaviour and often worse
I did a small movie last night,
http://www.youtube.com/watch?v=cEByKPPRhRM
from 00 to 0.10
You can see forward working ok (it doesn't no work so well when the car is on the ground because it often cog badly right in the middle of an acceleration)
From 0.10 to 0.31
I'm trying to go backward with different level of speed and show some bad cogging no matter what.
From 0.31 to the end
Going forward and doing intermediate speed you can hear that at certain RPM it's isn't smooth at is sound a little rough a like a phase was out of synch around (0.47)
I did the same test with the monster max, and didn't run into such problem with it