Photos taken on Canon 40D are often 80% grey - and rest is clear!

Recently I took 100 photos in one session. On the display, they looked fine (at least I didnt see anything odd) - but when I downloaded it to my laptop, half the pictures would not open as thumbnails. And whatever program I used (on the damaged ones) would only show around 20% of the picture, rest was grey... almost as though chip in canon had not been able to capture all the image. And yet when I flick through damaged photos with Picasa, I can, for an instant, see the whole picture... and then it goes back to 80:20.

So 2 Qs:
1 - why does this happen? Problem with 40D, or flash memory, or something else?
2 - how come I can see the photo for an instant... and is there a way of recovering what was lost? Tried various applications, but none have helped.

Camera is 6 months old, nothing damaged, new lenses. Flash memory, 8Gb.

Any ideas? Many thanks in advance.

Felix