Event Studio Runs Agent as if first time at 2:37am a few times a week

[cygnex]

We have an agent that runs constantly (every fifteen minutes) and checks for data changes to a certain field.  This runs currently except once or twice a week the agent runs as if it is the first time it has run, meaning it returns all records in the table.  This agent does work as expected the rest of the time and has caught changes and returned only the record that has changed.  Is there any reason this may be happening?  It is occurring at the exact same time when it does occur but there is no other obvious pattern to it.