How to decrease RAM usage of SQL Server 2012 without restarting Service
App based on this SQL Instance is acting up, being Not Responding or working very slow


You have a performance issue that you need to investigate. Read How to analyse SQL Server performance for a starter.

Do not change random settings, like memory, in hope you fix the problem. Analyze, find the root cause and fix it.

You're probably missing an index or more.

For the record: DBCC FREESYSTEMCACHE('ALL')