Missing links that aren't missing

Hi all, 

I am continually getting a 'this file has missing links to image/URL' message when I try to export. But the links panel lists every link as correct. A manual search doesn't find any missing links. 

In addition, when I first open a file often the images appear as grey pixellated lines as though they aren't linked, but show up as linked with the correct path in the links panel. The file has not been moved or changed since last opening it and neither have any of the links been changed or moved. Update all links is greyed out, as the links appear to be linked. When I PDF the file it shows all links fine, but they appear pixellated or with white squares in them in the PDF unless I manually relink them all. Any ideas why this is happening all of a sudden (probably been a few weeks?) Very irritating.