This is likely the USB port failing on the phone. There are other symptoms, like dinging as if being constantly connecting / disconnecting, as well as never "going to sleep" and occasionally the battery icon flashing as if being charged.
There's a short somewhere, and I'm mad as **** - I've been looking forward to the 7.8 update, and my phone can't even communicate with the PC now.
I've had issues with mine starting back a while ago, though mine may be cable related (don't have a secondary microusb to swap). I've had good luck keeping it plugged into one of my back usb ports and essentially keeping it plugged in and moving it as little as possible.
Have not had any other issues with it though, as long as the cable stays put, i've been able to disconnect/reconnect on the phone end fine.