Any reason why those MP4 files should be problematic?

From the metadata, I don't see anything which would explain crashes on export. However, metadata don't tell what happens within the streams.

I'd suggest converting the footage with Handbrake to see if it fixes the issue. Here you'll find suitable settings:

www.lwks.com/index.php?option=com_kunena&func=view&catid=23&id=166802&Itemid=81#209009

Thanks for your help.

You're welcome. Did converting help?

I'll try it another day. Can I convert the files and then substitute them in a finished project - if the file names are the same? Maybe that's covered in the thread you linked me to- I haven't had time to read the whole thread yet.
Thanks again.

Yes, replacing files on OS level with Lightworks shut down should work. However, we don't know the cause of the trouble, which might well be damaged parts in the media streams, perhaps somehow skipped or repaired when converting. This could cause missing or damaged frames and wrong duration. Consequently, you should check the result closely after switching the material.

Converting the files with Handbrake as suggested did fix the problem - exporting to MP4 is much quicker and the pc is useable while it's going on in the background. Substituting the converted files worked fine, though if there are any errors guess I won't know until I view the exported MP4's, especially as I was using proxies (?). Maybe I should delete the proxies and remake them.
Thanks again for your help.

cjdirect wrote:

Using the proxies from the original media and exporting the converted is an optimistic approach. However, without optimism a man by the name of Christophorus Columbus wouldn't have started his first voyage across the Atlantic Ocean and Donald Trump would live in Kallstadt/Germany.

So, it worked out well for Kallstadt - not so well for me. I've re-made the proxies.
Kallstadt looks pretty