>... The problem is that sometimes, >it seems to focus pretty good. Other times, it's off.
If the focus issue is not consistent then I'd suggest two things, testing needs to be properly done and/or the issue is not with the hardware.
With previous models quite a few had to learn how to properly use the camera and problems did not exist with the camera. Understanding the focusing modes, how they work, how they work with each different exposure mode and how each focusing point works as well, are all applicable.
For me the D7000 was a big learning curve and setting up the focusing modes with the exposure modes to achieve the focus I expected each time, took a while to work out. Another issue for me was I am no longer able to hold the camera as steady consistently (as in every shot) as I used to be - thus resulting in some shots not focused while others were.