Unreliable pressure transducer readings
First disconnect the Vout from Arduino and measure it with a multimeter to make sure the sensor is fine by itself.  If it's weird without the Arduino then make a final check that V+ (while attached to the Arduino) is 5 V.  If that all checks out then you have deeper issues.

I've use similar Honeywell pressure sensors for projects and in one case I needed to amplify the output up to 10 V but because the input impedance on my amplifier was too low the pressure sensor output started oscillating.  I can't remember the details but I remember being surprised at how poor its drive strength was (it was easy to overload it).  So you might look at the circuit on the Arduino and make sure that Vout directly connects to the microcontroller without any extra loading (like resistors to ground or Vcc).