When the Sametime meeting server (the classic meeting server for Sametime 8.5x) is under stress, a problem in Microsoft Windows 2003 SP2 described in "TCP connections fail intermittently when both endpoints are on the same computer in Windows Server 2003 SP2 (Article ID: 979230)" can lead to the failure of current meetings, as well as the failure to launch new meetings.
To prevent this situation from affecting the performance of your Sametime server, apply the hotfix Fix307523 as indicated in the Microsoft article linked above. This hotfix is for Windows 2003 SP2; those customers not running SP2 must install SP2 before applying this hotfix.
As noted in the Microsoft article, this problem can occur when an application makes a TCP connection to localhost. For example, the Sametime server code uses localhost connections for communication between its modules. Under stress, a race condition in the TCP/IP driver in Windows 2003 can orphan these localhost TCP connections, causing applications like Sametime that rely on these connections to fail.
Restarting the system can provide temporary relief, until such time as stress leads to the next instance of the TCP/IP localhost failure.