brushless motor as generator

[flyinglawyer]

I am making a winch that will spool out line under tension.  I've had success using an old brushed motor.  I short the windings, so that when I unspool the line, the motor provides braking torque and line tension.  The old motor is not powerful enough, so I upgraded to a stronger, brushless motor with 5000w power rating.

Here's the problem: as I unspool the line from this motor, it builds torque until it reaches a certain value (130lb in my case), and then the current and torque suddenly drop to zero and then build up again, then drop again. 

The motor has 3 phases (which are shorted for the generator mode) and a 5-pin hall sensor connector (unused here).  Here's the exact motor: http://www.aliexpress.com/item/Electric-bike-brushless-dc-hub-motor-5000w-for-electric-bicycle-5kw-ebike-hub-motor/32234157960.html

Why is it dropping the current in generator mode?  Do these motors have some kind of a built-in internal protection?  I thought that there was nothing inside but windings and magnets (and halls), as this one uses an external 100Amp controller.

Any thoughts would be appreciated.
PS.  Does this 3-phase brushless generate AC or pulse DC?