w3wp process using 100% cpu usage in sharepoint 2010 server

We have a w3wp process running on the SharePoint 2010 server(Windows 2008 R2) which is using 50-60% of the CPU usage and showing 100% CPU usage in the performance monitor. Following are some of the finding with this issue

1. This process id is not listing under "appcmd list wp" command and not on IIS worker process. But I can see this process on task manager (only when I check on all user process) and can't delete by end process button.
2. I tried to delete it by using taskkill command and it shows the result that the process terminated but the process is still there in the task manager.
3. I have done IISREST and restarted SPADMIN service, but no use.
4. The process will stop only when I reboot the server.
5. This is not a regular issue on all days. This Process seeing in the server once or twice in a week.but not on a specific day.

Please help me to solve this issue.