[Answered ]-Celery and Celerybeat randomly start acting funky

Your settings look good.

Could it be that you are measuring when the task is completed, not when it is starting? It could be that your tasks take very long or even fail so they do not get reported.

crontab is low level and fairly accurate in my experience, so I’d check your logging tool first.

Update

Another debugging solution is to start implementing time limits for tasks to make sure they do not get locked too long. http://docs.celeryproject.org/en/latest/userguide/workers.html#time-limits

CELERYD_TASK_TIME_LIMIT = 300
CELERYD_TASK_SOFT_TIME_LIMIT = 260