Uno dropping serial port connection

Hey guys, I'm having some weird issues with my UNO.

I've been using it for a couple months now without issue, but within the last few days it has started behaving oddly.

Sometimes, when I plug it in (esp. if Arduino is already open), or if I reset it using the onboard button, it will disappear from the serial port list in arduino and I cannot upload anything to it. I can, however, still see it in System preferences and device manager, but all the information says "Not available".

Sometimes it connects just fine, but when I try to upload a new project, it gets stuck and then drops the serial port connection. Usually when this happens it does not show up in system information at all. I hear the little disconnect sound when this happens, then I'll hear the reconnect sound, but it won't show back up in the serial port information in the Arduino program or in system information.

I'm not sure if this is related, but it started doing this when I began using an Adafruit Motorshield v2 with it. I'm barely able to get any of the motor tests to work because of this issue.

I'm having a similar issue with my Arduino Uno while trying to run a long-term voltage monitoring setup. The IDE isn't being used - I have a little C++ program that sends a signal to the Arduino and then dumps the data the Arduino writes to the serial port into a file. The program will run perfectly fine for a certain amount of time (not sure exactly how long, as I've only managed to reproduce it today and without a timestamp, but more than a few minutes and less than three days) but eventually the Arduino just falls off the face of the earth and has to be manually removed and replaced before it will show up again.

Very confused about this and rather concerned, it impacts the viability of the entire setup I've been working on.