The only reason I didnt think it was a gyro/servo issue is because you can literally hear the motor power cut for a millisecond when it happens.

When the heli is under power...the gyro is constantly trying to keep the tail held.  When power is lost to the motor, even for a millisecond, the headspeed will decrease a bit...and the gyro will have to compensate for this.  Even though it is almost instant...the tail will have no other choice but to show some type of wag/reaction to the loss of headspeed (which directly effects the speed of the tail rotor).

At least that is what I got from the video anway.

I didnt really notice it till I got a few seconds into the video...dont know if it was exactly a "minute in"...but just thought that if the problem came in after about a minute of spooling up...that would be ample time to heat up motor/exc/batteries to generate enough heat, for heat to be a factor in the symptom shown.

I really dont think its a battery issue...although I guess a poor solder job on a connector "could" cause an issue.  But I would think a bad cell or connection would give low/no power at all times...not just intermittent.

At this point...I really dont know.  But I still think looking at the ESC/motor is a good place to start.

Exceed Alpha 400 and Pentium/Hobbywing 40amp ESC are very cheap at HobbyPartz.Com.  Could buy both for probably 40 bucks or less.  And if its not those items...not a bad idea to have a spare motor/esc in the parts box anyway.

goodluck