Douglas, the usb connectors on the Pluto are not of the highest quality and if you do enough connecting/disconnecting you will find they start to fail. 

It is a very good idea to always have the second usb power-only connection in use at all times. This helps alleviate a lot of problems.

Also, not all usb cables are equally good as far as the quality of their connectors. Some certainly have a better, more solid fit than others.

And finally, replacing the on-board connectors is not trivial, unless you have very good smd skills and equipment. 

regards, Roy