i just want to clarify when the stepper motors go 'slack'
they will move in the specified direction but it's stilted and every 5-10 seconds the motor relaxes the projectors falls to a balanced position then move again
I agree with grumpy_mike, the symptoms look like thermal cycling. The motor controller overheats and protects itself. After the temperature falls, it restarts.
If you reduce duty cycle do the motors return to normal?
Joe