I would say that adding two extra connection points and an extra adapter circuit introduces more reliability issues than simply sticking with one cable. By using a USB to Ethernet adapter, not only have you failed to eliminate reliability issues with USB (after all, you are still fundamentally dependent on USB) but you have added an entire second cable with it's own possible issues. Reliability isn't necessarily defined by what connector is going into the CP4/5.
Windows does have USB power saving settings, although I don't think they would cause this issue. You can check easily by simply turning off the selective port suspension in the settings. This shouldn't affect the CP5 however, since the FTDI chip running the USB is powered by both the CP5 and the computer for maximum performance.
Do you have the FTDI driver installed? Windows will find a driver but not necessarily the right one every time. You can find those drivers here: https://ftdichip.com/drivers/
One troubleshooting option would be to check that the digital board of the CP5 is fully seated on the main board. Power down the CP5 and use a 3/32" hex key to open up the front cover with the antenna. Inside there is a small circuit board on top of a large circuit board. Press down on the side of the small circuit board closest to the connector panel and see if it seats into place. If so, that might be the cause of your issue. I doubt this is the case, but it isn't a bad place to check.