SQL Server RAM usage is increasing and query executions is slowing down with time I'm using SQL server on a server with 48 GB of RAM. 

When I start the SQL Server service, it's using 12% of the RAM at first, but after 24 hours it's reaching 90% and query executions begin slowing down. We have to restart the service so we can serve users. 

What could be the problem and how we can fix it?

Example images:





and 
(select value_in_use from sys.configurations where name = 'max degree of parallelism')



and my server cores are 47.
Based on the waits statistics in your system you have incorrect settings for parallelism.

Maxdop in your system should not be more than 16, you can find the correct value based on this MS article: Recommendations and guidelines for the "max degree of parallelism" configuration option in SQL Server 

Here are some T-SQL scripts to do it (I don't know your exact NUMA configuration but the scripts alilyze it):
MAXDOP setting algorithm for SQL Server


Here is the explanation given by Max Vernon on which I agree. He explains why incorrect setting of maxdop slows down the execution.


  If you set MAXDOP higher than the number of cores/numa node, you end
  up with calls into far memory which are many many times slower than
  calling near memory. This is because each numa node has its own
  memory; having a query use more threads than are present in a single
  numa mode will spread the CPU load over multiple cores, and therefore
  multiple memory nodes.