I'm not complaining there is a problem (used both bodies a lot got my money's worth out of them and then some), I was just curious as to what the issue is. There is an interesting development I have noticed...that is if I take a second shot straight away afterwards it's fine, I also tried it in cont. drive and again images were fine.
My thoughts are it's something to do with the first black frame problem (this was the camera I bought from Australia for not that much a good while back), my original 5d was sent in and fixed for free and I've not seen any problems with it since. Looks more likely to be related to that shutter/mechanism, I'm sure DK would have an idea if he's around. I'll still use it, was just curious about why it did that