SQL queries failing, Memory not releasing
SQL Server is kind of a memory hog. You could try clearing the cache, BUT I would highly recommend doing some reading on these commands first before you using them. From what I hear, their can be negative side effects when using these especially in a production environment.

DBCC FREESYSTEMCACHE ('ALL')
DBCC FREEPROCCACHE


Here are some links I found useful:

Difference between FreeProcCache and FreeSystemCache

https://sqlserverperformance.wordpress.com/2009/12/28/fun-with-dbcc-freeproccache/

http://jamessql.blogspot.com/2012/04/clean-sql-server-cache.html