With the car off and the key on the ON position, the gauge jumps around for 20 seconds then settles at 15.5. I don't remember it jumping around that much with the car off.

 

Does it stay at 15.5? Sounds like the sensor is on its way out. If you turn the key to on without starting the car the gauge should bounce around a little then slowly go over to full lean and stay since if the car is off its not reading exhaust just oxygen.

 

Brand new sensor