Random 500 / Internal Server Errors

[sweethome-il]

hello,

I have moved a rather large site from H-sphere / windows 2003 server to Windows 2008 R2 with Plesk 11.09.
Site uses ASP & .NET with a MS-SQL 2008 DB on a remote server.

Since the migration, clients are getting sporradical 500 / Internal Server Errors from random pages in the website.

I see no application pool crashes in the logs, and the 500 errors are NOT logged in the site access log, but from the look of the page, it definately comes from Plesk.

Please advise how to proceed with my investigation.
Do you think the issue is related to a Plesk configuration or a Windows misbehaviour?

thanks,
Justin