Check the XLR connector on the base of the microphone. Look for loose pins. The capsule of the microphone in the main body is a sealed unit so any instability is likely to be a result of loose connections in the XLR connector.
Loosen the recessed screw on the side of the microphone body using the screwdriver. Access the Allen bolt underneath the screw, loosen and remove it. This will release the connector housing, allowing you to slide out the connector body and view the soldered contacts on the back. Check all the joints are solid and secure.
Check the wires are connected properly. With the locating pin pointed downward to 6 o'clock, the four electrical contacts should have wires connected in the following order: red, green, blue and black.
Check that the red wire is connected to the correct impedance pin for your system. There are three labeled pins for 50, 150 or 250 Ohms. Move the red wire to the appropriate pin if your RE20 is set to the wrong impedance.