Debugging of a custom module which basically works (computed field based)

A programmer has made some modifications on a content type (CCK) so that images are fetched from a remote web site and are stored into calculated fields ([login to view URL]) with PHP. The parsing og the remote html is done with the simplehtmldom module ([login to view URL]) but i guess you don't have to care for this because the module basically works as it should so the "main program syntax" ,ust be okay. This means: The images are saved and the node can be viewed, but after some time those images disappear. There is a wring image source then stored in the field. It is a directory then instead of an image ressource. I have no Idea why it happens and when it happens. It does not seem to be related to cron runs, but it I am not 100% sure. The task is: Make sure that the data / content / value of the content type is stable and does not disappear.... Maybe its a small bug and done in some minutes..... Maybe it needs more time to test and investigate..... What I know is that the module is not large (not many lines of code)..... There are 9 fields that contain this code snippet to be automatically (re)calculated. One of those fields holds the collection of the images that disappear. So the task is to debug the code of this field(s). This has to be done in a drupal compliant manner.