Pushing shutter button half way down to lock focus question

I dropped my Pentax k2000 and had it repaired by Pentax.  Now it appears that when I have the camera in auto-focus mode and try to push the shutter button half way down to lock the focus, the camera won't keep that point locked in if I move the camera to another object to photograph.  Essentially, the camera continues to rapidly re-focus on whatever is in the view finder.

Is there something in the menu's of the camera where I can force the camera to lock focus when I push the shutter button half way down?

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Pentax k2000 camera
Pentax 55mm-300mm lens