You'll have to investigate how its final polishing is done. If its due for preventive maintenance, then do so. If not, consider that it may still need to be checked out by a service technician. It may be that your system has some sort of intermittent microbial growth in some dead space within it. Also, intermittent trace results are also suspicious -- you may be contaminating your receiving vessel, or your transfer tubing, or may even lack some accuracy with your TOC determination.