Correct path replaced with /tmp

I have been using FileField with ImageField for a while now, generating roughly 100 nodes with images.
I've noticed recently but only occasionally that if I edit an existing node, the image is missing in the edit page and when I save the node, the image is gone from the node, though it remains in its proper location in the file system.

I have the file path in filefield set to: resident_child/[nid]

Using the Node Export module, I exported two nodes: one with the problem and one without.
The good file contained this line:
'filepath' => '#FILES_DIRECTORY_PATH#/resident_child/195/IMG_3481 (Medium).JPG',
and the bad file contained this line:
'filepath' => '/tmp/IMG_3479 (Medium).JPG',

The imagecache generated thumbnail used in a view of the nodes also had the /tmp corruption:
http://mysite.net/sites/mysite.net/files/imagecache/Scale-065pxWide//tmp/IMG_3479 (Medium).JPG

I've found these clues but am not sure where to look next.
What could be causing the /tmp substitution?

Installed:
FileField 6.x-3.7
FileField Paths 6.x-1.4
FileField Sources 6.x-1.2
ImageField 6.x-3.7
Imagefield Crop 6.x-1.0-rc2
ImageField Tokens 6.x-1.0

Thanks
Steve