System Memory (RAM) for Windows Server 2012 Standard with SQL Server 2012
All of this assumes you're running 64 bit Windows and 64 bit SQL Server...

SQL Server is greedy when it comes to memory usage, and that is by design. It keeps as much data as it can in RAM because it is faster than having to access disk to fulfill a query or other request.

As a result, you may want to set the minimum memory allocation for a SQL Server instance so it doesn't suffer from what I call memory starvation. If this happens, the server's drives will thrash because database usage will require disk accesses. 

You'll also want to set the maximum memory allocation as already mentioned. In general, I give 10% to the operating system and the rest is for SQL Server (I have dedicated SQL servers each with 32GB RAM so it's fairly straightforward).

This article from Brent Ozar's website has more details: https://www.brentozar.com/archive/2011/09/sysadmins-guide-microsoft-sql-server-memory/

I noticed you said sometimes the performance suffers. Do you have other applications collocated on this server? Are there SQL Agent tasks scheduled to run at certain times? Backups can take a lot of resources and other applications sharing resources could end up fighting over them with SQL Server. In that case, you may need to add more memory and carefully tune how it's used, or move the application to another server.