Is it possible to define an performance counter and thresholds which can stop an ongoing circular trace? Other possibilities to unattended stop of a trace?
Have you checked performance monitor alerts? You can define them either using logman.exe or in perfmon.exe. Performance alerts constantly check the specified performance counter(s) values and execute actions when a defined threshold is reached. You can find a tutorial how to create one here: http://technet.microsoft.com/en-us/library/cc722414.aspx.