Select query gives insufficient system memory.

Issue: Msg 701, Level 17, State 123, Line 1
There is insufficient system memory in resource pool 'default' to run this query.

Details : Some times we are getting the above issue when we run the select query. Please give us heads up why this is happening, what is the cause  and what could be the resolution.

SQL VERSION is: Microsoft SQL Server 2008 R2 (SP2) - 10.50.4000.0 (X64)   Jun 28 2012 08:36:30   Copyright (c) Microsoft Corporation  Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) 

Memory out of 8GB RAM we set max memory to 5120 MB is set to default, the error is happening here in default instance.

there is 1 named SQL instance and SSIS, SSRS running on this server.

Please help me in troubleshooting this and to resolve this.

Thanks.