Sorry, lost my earlier post because our web filter is terrible. Here goes at another attempt.
Post from 2010 with a similar situation:
https://www-secure.symantec.com/connect/forums/altiris-ds-must-be-restarted-often
Solution which was linked there from 2005 (updated in 2006):
http://www.symantec.com/business/support/index?page=content&id=TECH10489
Same solution provided in 2011:
https://www-secure.symantec.com/connect/articles/deployment-console-why-are-computers-offline-troubleshooting#comment-6190531
Basically, if you have this sort of thing happen, one of the major reasons is SQL database bloat. Especially if you have recovery mode set to Full or Bulk, you can have your transaction logs expand to huge levels - one person posted 32 gigs of database of which 31.9 gigs were their logs. The KB article suggests changing your database recovery mode to Simple unless you actually want to properly maintain it.