Well this can happen under certain circumstances when shooting a scene with difficult to distinguish backgrounds and can be due to the lighting conditions at that particular time, if the auto focus definitely works OK indoors its liable to be an exposure problem that is upsetting the system even if its on the normal AF position, (assuming its on this) so what happens if you set the exposure to manual? (adjusting this if necessary) but leave the focus on Auto.
Another point of interest being, does this happen in a cold or hot environment and does outdoors mean normal daylight?
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