It looks like you need to set the Maximum Server Memory to a lower number -- that setting defines (roughly) the maximum amount of memory that SQL Server will use, which should be most of but NOT ALL of the actual memory in the physical server (if this is a virtual machine, I may defer to others, but first I'd suggest that you confirm that the host's memory is not over allocated). And by and large, SQL Server will use the entirety of the Maximum Server Memory -- any excess not needed for computations will be used to improve performance by, for example, caching tables.
You do not state the total amount of memory in the server, but:
(1) you should always leave enough memory for the operating system and any other services (how much? opinions vary, as these links show, but I'd suggest you try at least 4 GB: https://www.sqlservercentral.com/blogs/suggested-max-memory-settings-for-sql-server-20052008 and https://www.brentozar.com/blitz/max-memory/ and
(2) however much you have left for the operating system and other services right now, it sounds like it is not enough. Try dropping the maximum server memory by 1 GB and see if that helps. SQL Server will adjust to manage itself within the lower amount of memory.
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