D7000 inadvertent AF mode lock out

for lack of better term.. I've had my body for about four weeks now, and for about a week, the camera gets locked in to manual AF mode without me doing anything. This is happened with some of my latest lenses (24g and 16-35vr)which has no such issue on my D700. When this happens, I have to reset the AF/M switch on the camera to engage the autofocus mode again. Anyone else with this trouble? Any ideas what I could be doing wrong?

I am also suspecting back focus issues, but have not yet tested to confirm.