We have had several issues with intermittent performance & timeout issues on SQL 2008R2 servers. Most recent is a 43GB database in shared environment with other smaller db's with about 250 concurrent users , SQL Server had 4GB of RAM. DBA's ran metrics that indicated page life expectancy was way over 300 so memory will not help. After 2 days of issues RAM was increased to 8GB and issues disappeared.
Had identical issue with a 150GB database on SQL 2008 with 2GB of RAM - page life expectancy was good but when RAM was added performance improved significantly. Can anyone answer why page life expectancy indicates memory not an issue but adding RAM resolves the problems ?