Hi Dan,
I have seen similar issues when the job has been scheduled to run on specific dates AND if too many dates into the future has been selected. Could this be what you are experiencing?
You start "Presumably it is listed in Job Monitor until shortly before the scheduled run?" - do you actually see the job as scheduled in either "Job Monitor | Job List" or in the "Job Monitor | Calendar? If the job is not seen as scheduled, have you tried re-scheduling it?
If you have not scheduled your job to run on specific dates, then there could be something either with the database or the job itself. You could try running a "Rebuild Database Indices", a "Repair Database" and a "Compact Database" using BEUtility. However, if the problem is with the job itself, you might have to get the job manually delete from the database and recreating it. This would require Technical Support to be involved.
Let me know how it goes.
Regards,
Christiane