I'm assuming that when the server becomes very slow to respond to requests that the CPU usage drops to 0%. During these times check the response times from your server by doing a ping. It could be a problem external to your server. Check the response times compared to when the server is running fine.
When the CPU usage is greater than 0% what processes are using it most? It could be that the main IIS process has failed and is restarting hence the drop in CPU usage. Check the Event Viewer logs as they may be able to tell you what's causing the problem.
This must be a IIS bug.
When the CPU usage is 0% I can access the server by Terminal Service... everything works but IIS.
Nothing in event viewer... just a warning about SMTP... failure delivering..
Did you see here: http://www.patroni.brturbo.com/
This show what happening... the server is working fine and stop to answer.... it doesn´t become slow before to stop... it just stop... and come back.. like this pictures showed...
hey, the same thing happens to me a lot. but in my case i think its usually because of some errors in my code (especially querys etc). i would check the code again just to be safe, otherwise it probably is a problem with IIS.