Spoke with 3 people at Apple support. None able to identify the problem although the last person was able to help eliminate a few items - i.e. created a new user and tried (failed) so its not a corrupt user; created a new iMovie library and tried (failed), dragged photo's to the desktop and then into iMovie (failed) so no issue with media import etc - all suggested changes had absolutely no impact on the problem.
I was able to demonstrate that even 2 photo's from the same camera taken within 10 seconds of each other behaved differently in iMovie once added to the timeline.
His next suggestion is to use the app "Image Capture" to see if that helps, and pending that outcome the only other suggestion (at this stage) is to uninstall iMovie and download a 'fresh' copy and reinstall.
Option 1 is a workaround and doesn't address the issue. Option 2 is just hoping..... Not an encouraging outcome but I am sympathetic in some way of the task the Help staff have in front of them. He was very keen to resolve the issue but I suspect he was still a 'generalist' in terms of support and that a programmer will be required to debug the problem - if they can replicate it in the lab.
One thing of interest (to me at least and may not be related) is that the old iMovie app was about 420Mb in size. The new iMovie is over 2Gb, so a lot more complex? As I was closing down the iMovie app and just chatting to the Support guy, I noticed it was very very slow to shut and in fact displayed a message that it was attempting to close background processes or similar - even though it wasn't rendering or adding media or any other function that could conceivably require the app to be 'engaged'.
This delay in processing was also evident when testing creating a new movie. I added 6 photos to the timeline and tried to 'playback'. Initially all 6 were blacked out in the playback screen (top right hand corner of iMovie). As i continued to talk to the tech I played the same photos back a few times. Within a couple of minutes the 2nd photo suddenly behaved perfectly in playback, whilst the remaining 5 were still blacked out. Then a few minutes later the 1st photo also started to behave as it should.
I then waited 15 minutes but they remained the only photos that behaved.
So, i wonder if in the background the system is trying to render or process the images but for some reason either my computer and ram/processor are just not fast enough to allow that OR there is a bug in iMovie that puts it in some sort of loop and doest allow it to finish processing the photo's?
So, no answer and still a complete mystery.
If anyone has tried the ImageCapture option to import their photos and/or reinstalling the app then let me know so I don't waste any more time.
Thanks