Sometimes running out of Buffer Pool SQL Express 2014

I am experiencing an intermittent problem with SQL Express 2014 x64: I have a database with a "fairly" large table:
TABLE [dbo].[QST2D]( [REF] [char](36) NOT NULL, [QST_Num] [tinyint] NOT NULL, [NodeID] [smallint] NOT NULL, [ValueID] [smallint] NOT NULL, [NodeVisible] [bit] NULL, [Points] [smallint] NULL, [QST_ID] [smallint] NULL, [Value] [nvarchar](max) NULL, [ValueVisible] [bit] NULL, [ParentVisible] [bit] NULL, [bvalue] [int] NULL, [bvisible] ...