Golden Motor BLT behaves strange

Can someone please help me with this issue:
I have just built two golden motor blt-800 into a cargo bike. I use the Golden motor "cruise" controllers.
When the motors get some load they start behave strange: it seems like they periodically stops for a millisecond. I am 95% sure that it is not a mechanical issue.
I tried to make a video, se https://drive.google.com/open?id=1waLPS67R9aCt6SVczi0mk_Y9PtaGKdcA. In the video I hold the brake handle to make a load to the motor, and I let the motor spin relatively slowly so you might see that the wheel stops up periodically. It happens the both motors. It makes a loud bang-noise. One reason is that the cargo bike has a aluminium box which amplifies the sound:-/ (so this issue might not be so obvious when these motors are used in a normal bicycle frame...)
The controllers are restored to fabric settings (30A continous, 120deg phase angle, see attched screenshot). Battery voltage is around 50V

What can the reason be?? Is it just a crappy motor and controller?

Have you played with the settings, like put it to 60 degrees. Or swap it to reverse direction. Also the voltage is only set at 24v. It reminds me of a hall sensor out or a phase wire mismatch.

Thanks for your comments xlr8!
I have tried 60degrees and reverse, and I think i also tried different voltages 24/36/48V, but I will try altering the voltage settings again!
So if I understand you correct, there might also be something with the hall sensors..? I have this "ebike tester" from em3ev; https://em3ev.com/shop/ebike-tester/ Have not used it before, but maybe I can hook it up to the blt-800s and do some testing...

I am also considering to try the motor phase calibration described for the motor controller in the "manual" (but as I understand it should not be necessary for motors from Golden Motor:
8. Motor phase Calibrating (Do not use it unless your motor is not from GoldenMotor):
The controller can work brushless motors with different phase angles other than defaulted 120 degree.
Below are procedures to let controller match the motor phases:
zCut-off power to controller and motor
zGrip and hold power brake lever, turn throttle to maximum speed position, depress the cruise control
zbutton, turn on the power switch (using battery key)
zConnect battery power to the system, release throttle, release brake lever, release cruise control
zbutton.
zListen 2 beep sound, turn the motor wheel forward and reverse once
zIf hear 1 beep sound, motor phase calibrating successfully completed.
zOtherwise, swapping any two of three motor phase wires, and repeat above steps.

Another strange thing: at default current setting 30A/50A the bike feels powerful. But when I lower the current to 15A or even 25A, the motors seems to be relatively weak (compares to a 250W e-bike). I will put in a current shunt and do some real time measurements as well.
But anyway, is it not strange that at 15A*50V=750W for each motor, the bike feels weak?

I was 95% wrong! It was something mechanical. I have to make new motor brackets...