Occasional High volume Session

Does any body have any idea why this could be happening for us ?
We have a scheduler running which runs 4 AM everyday morning. Occasionaly once in a month, the Session Sweeper, Remote server sweeper, Statistics log, DB Session sweepr, Remove Expired cluster Session, Template catch sweeper in server administraor goes to a NEGATIVE value and after that the number of sessions in our environment goes way high. The high value of NUMBER OF SEESION which is around 1185, forcing us to server rebooting. Is there a configurable Server paramenter to overcome this situtation ???