dwc2 ffb40000.usb Overcurrent change detected

I get the message when writing to OTG/USB (de0-nano-soc)

dwc2 ffb40000.usb Not connected

dwc2 ffb40000.usb Overcurrent change detected

-- then my attached storage is gone till the next reboot

- while investigating i have removed the supply to the USB device & am powering externally

therefore there should be NO over current (current to external device is <100ma so should still not have an issue )

there are no devices being powered off the board - only load is the fpga & support circuitry on the board

(running both processors {linux kernel 4.4.0) , one instance of NIOSII & one of msgdma)

works fine when writing to local SDcard...

next guess is a heating issue - as it seems to function while cold but within 5-10min it fails as above .. ..

any hints ? should i be looking at heatsinks & fans (uuggh) ?