AppleUSBHostPort::interruptOccurred: overcurrent detected

Hi,

I receive notification alerts: "USB Devices Disabled - Unplug the device using too much power to re-enable USB devices."

I get these notifications whether I have genuine Apple products and genuine cables plugged into any of the USB ports or even if I do not have anything plugged into any of the USB ports. I can receive up to four of these notifications at once. If I dismiss them, they return within a varied amount of time. Sometimes about an hour later.

The system seems to have also slowed down due to this issue because the kernel_task process seems very busy logging this error to the Console more than once per second.

The following is being logged in the Console for Kernel activity:

"AppleUSBHostPort::interruptOccurred: overcurrent detected"

I have an iMac 5K, 4Ghz i7, 16GB RAM running El Capitan 10.11.2

Steps I have taken:

1. I have reset the System Management Controller (SMC).

2. I have reset the NVRAM.

Any ideas on fixing this issue please?

Thanks.