Why BCHR doesn't normally matter: https://www.simple-talk.com/sql/database-administration/great-sql-server-debates-buffer-cache-hit-ratio/[/url]

As for PLE drops, you can trace for queries running when the PLE bottoms out to figure what is happening in the database when it drops. Many times it's a nasty select for a report that requires a ton of data which displaces a large percentage of the buffer pool. You can also look for external pressure, i.e. memory set trims imposed on SQL Server by Windows. You can find notices for those in the SQL Error Log.