Disapearing pl2303 usb serial adapter on rpi2

[bob prohaska]

On an RPI2 at r326951 a pl2303 usb-serial adapter seems to be locking up 
after a few hours. In the past this could be rectified by using usbconfig
to power cycle it, but now even that doesn't work. Apart from the serial
device file vanishing from /dev there seems to be no other ill effect. 
Rebooting fixes the problem for minutes to hours. 

There is no explict error report in dmesg, but when usbconfig tries to power
cycle the adapter the console reports:

uhub_explore_handle_re_enumerate: Could not unconfigure device (ignored)
usbd_req_re_enumerate: addr=9, set address failed! (USB_ERR_IOERROR, ignored)
usbd_setup_device_desc: getting device descriptor at addr 9 failed, USB_ERR_IOERROR
usbd_req_re_enumerate: addr=9, set address failed! (USB_ERR_STALLED, ignored)
usbd_setup_device_desc: getting device descriptor at addr 9 failed, USB_ERR_STALLED


A second RPI2 running r326747 is subject to the same lockup, but the adapter
can be unstuck using usbconfig to power cycle it. 


Are there any diagnostics which could shed more light on the problem?

Thanks for reading,

bob prohaska