EOS 450D – frequent fatal fall foliage focus failure

I would value some advice please – interpreting the cause of the failure of the pictures I took on a recent trip to New England for the fall foliage.

Sadly the majority of the pictures seem to be out of focus.

First – settings / lens / circumstances:

The majority of the pictures were taken in:

aperture priority (Av) mode,

ISO around 200,

using my EF-S 55-250 at near maximum zoom,

in overcast conditions,

evaluative metering,

one shot autofocus,

single central point selected for AF,

lens set to AF (of course!).

Image quality: medium size, highest quality.

During shooting there was nothing to suggest a focus problem, and images appeared sharp in viewfinder and on LCD display. I was usually focusing on a tree in the centre of the frame, at some distance.

The pictures below have been resized in Irfanview to 40% for purposes of reducing upload file size.

2 full size images (IMG_2957, IMG_2986) have been uploaded to my gallery:  http://www.dpreview.com/galleries/8211691469

The outcome was usually as shown here:

Sample - focus "soft" throughout

I have subsequently tested things at home, with 3 lenses - the same 55-250, the EF 50 ("nifty 50") and the kit lens (EFS 18-55). Settings the same as above. The results demonstrate similar problems. In each case I was focusing on the pale green ball shaped conifer in the centre of the frame:

Kit lens:

kit lens

Nifty 50:

55-250:

To my eyes, there is a failure to focus correctly - and given that I have had satisfactory results previously with this camera and lenses, I am at a loss to understand the cause!

Finally, I took a sample picture of local foliage at a distance of about 75 yards - it appears that a similar problem of incorrect focus exists:

Sample image (larger file, maximum quality setting)

So - if there is an explanation for this problem, I would be most grateful for answers, and suggested solutions.

Many thanks.