Freeradus service goes down

Dear, I have a Freeradius server running OK, authenticating against an AD. The Debian package is 2.1.12+dfsg-1.2 version. The problem is that the service goes down 2 o 3 times a week, and I can't know tha cause. I see the /var/log/freeradius/radisu.log but there is nothing important I think. When I start the service in this way "service freeradius start", today the service didn't start, and I have started it with "freeradius -X" and it runs OK now. How can I find the origin of the problem the service goes up frequently??? Do you recommend to me start the service with "service freeradius start" or "freeradius -X"??? Thanks in advance. Roberto