Why does my oil pressure gauge drop

It sounds like you have a grounding problem in the dash. Look for grounds, of course they will be black wires going to a clean metal connection, and check them for a clean tight corrosion free fit. In the mean time you might want to go to an Advance Auto or Auto Zone and borrow, they lend out free tools, a oil test pressure gauge to make sure your pressure is okay. The sensor may be bad but from you describe with the switch changing it it is probably a shared or co-located ground between the motor and the gauge and/or switch.