When I am taking a shower, the water sometimes cuts off. There is a noise, (honestly kind of like a loud fart, sorry), and then the water cuts out. I turn the middle selector back to the tub, water flows freely. Wait a minute, turn it back to shower, and usually cuts off after the noise. How can I repair this?

Could be loose washer in diverter valve (middle selector).

The Widget Maker's on to something, The problem sounds like it's in the diverter,(see image).
Remove the knob and open up the diverter . Check and replace worn or torn washer and check the seats for nicks or rough spots. Now, while the valve's open turn on the hot and cold water for a few seconds to flush out any debris that remains, reassemble and test. Better now? Good luck, Tom