Curious 36120 problem

[Gustav]

Got my LST running today (thread to follow with build pics and details),anyhoo,got a problem with the 36120.setup is 36120,1950/10(1307kv),8s lipo.
Basically it cuts the motor hard at a certain rpm,it's definately nothing to do with the truck,ubec,radio,lvc etc.It does it in forward only or in forward/brake/rev. mode.

It cuts so hard that out of the truck the motor jumps about an inch.I'm having to run a high load to stop it happening,anything below 11:1 final drive on 8s it will cut at WOT/high rpm.This is in a 13lb LST so it's a fair old load.I can't see that it's hitting the 42K rpm limit since it will even do it on 16 cells unloaded.Is it possible for 16 nearly flat cells to send a 1307kv(loaded rating) motor to 42000rpm unloaded? I seriously doubt it (that would need around 2200 unoaded kv)but i guess that is the question.Any ideas WTF else it could be? It runs great if i keep enough load on it,smooth,cool and seriously quick.