This sounds like you are running other applications on the same server as SQL server. This is generally not considered best practice because of the way that SQL server works by using as much of the memory cache as possible to speed things up.

I did however work at a company that had to work within the same constraints as you in so far as having to run multiple applications from the same server. The best thing you can do is make sure that you set the max memory usage in SQL server so that it doesn't take over memory that should be used by other applications.

In closing, the options you have are either limiting the max memory usage in the settings of SQL Server, increasing the physical memory (this will eventually run into the same problem if you don't limit the max memory), or move the other applications to another box.