Automated process not running

we have a few process that are suppose to run at different intervals through out the day.  They just seem to stop running for no reason that we can find.

If we reboot the server or restart the server extensions they come back and run for a undetermiend amount of time. Some times an hourly process will run for 5 hrs after a reboot or restart of the extensions sometimes it will run once it is all over the place.

Has anyone seen this?

Thanks