I could deal with that if my D300 just worked as it was supposed to. Unfortunately, despite having already been back to Cameta/Nikon for servicing, I think they just put it in a circular file and returned it to me after a month, having done nothing to fix it.
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Nikon tends to put a repair report in with all their repairs that say what they did to the camera. If you didn't get one of those, then I'd call up Nikon and see if they have a repair record for your camera's serial number.
Just today, I was taking some sports pictures, and my lenses kept backfocusing and sometimes not even focusing at all in the actual image, despite the camera being focused in terms of what I saw through the viewfinder (i.e. person in focus in viewfinder, person out of focus on the display after the shot). I took several shots in a row of someone in the outfield that was standing completely still in bright sunlight (i.e. fast shutter speed) and the camera repeatedly gave me out of focus images. I couldn't post them even if I still had them because the subjects are minors, but I hope you can take my word for it. Other shots gave me similar results. I used both C and S focus modes, and fiddled around in general, but the results were always inconsistent at best.
What happens if you manually focus? Are you using the Nikonians settings, the default settings or some other combination? Closest subject priority set to off? Nikon's advanced autofocus modules are pretty tweakable, and it's possible to mis-set it and get bad results. You should be able to repeat the results with subjects you can post.
Frustratingly, sometimes my D300 works as it's supposed to. I feel like I'm good enough to know when it's me and when it's the camera, and I'm almost certain it's the camera, particularly as it's already been found in need of service once before by a professional and sent back to Nikon.
In most of the cases I've seen, when things are inconsistent it's a setting problem. If I were you, I'd rule that out prior to completely giving up.
At any rate, I love the D300 for what it can be, but I'm not too fond of my actual D300 these days. It has around 14,000 clicks on it now, and given that I can't actually be sure if it's in good technical condition, I don't even know if I could sell it.
Send it to me, and if it's not good, I'll send it back
Anyone willing/able to talk me off a ledge here?
Check all the AF settings, make sure you understand what they do and how they're set and how that affects things like moving children.