autofocus – Nikon D750 erratic focusing

I hav been having focusing problems with Nikon D750 .

When it tries to focus on a subject, it will take about 3 or more seconds to try and focus. I can hear the servos move in an out of focus several times before it will finally focus.

This is especially annoying when I am trying to film and it will just go ballistic on a moving subject on full-time servo. Some times I hear it move in an out of focus on a tripod, nothing is moving in the shot.

It will also sometimes try to focus on a subject that is within the focal range when the shutter button is half pressed, move the focus in and out a few times, and then the focus square just flashes red.

What is wrong? Can someone please help me with this?