I have no way of knowing what is causing the issue, be it hardware or firmware. Unfortunately, AMLogic has not released new SDK for the S912, so no new firmware exists, as developers depend on new AML SDK to make improvements.
You may want to try superceleron’s custom ROM, it offers improvements over stock firmware.
As you have already tried a different cable(power), and a different power supply, flashing firmware may be the only solution left before hardware fault becomes a consideration.
For testing, disconnect unnecessary USB devices to insure they are not to blame.