Unlike Win XP and older OS's which limited the number of active threads for a running process to 2024, Windows 2003 and higher only limit the number based on the availavble resource, It is possible that threads are not being closed as fast as they are spawned and therefor you will see this limit hit. Check you system and Application event logs to see if there are evenbt messages posted there. Lastly make sure you do not have applications launching at those times that do a burst consumption of ram.