Focus on the following areas:

1)OLE Automation usage that is causing VAS pressure\fragmentation

2)Excessive Linked Server Usage

3)Custom Extended Stored Procedure

4)3rd Party Backup Application (such as  Redgate SQL Backup) using to large of a Network Packet Size.

5)Your applications are connecting with a Network Packet Size exceeding 8KB and going into VAS/Multi-page allocations. 

6)Adhoc (non-parameterized) workload leading to  plan cache expansion

7)SQL CLR usage that is causing VAS Pressure\fragmentation

Potential approaches for the above are:

1,2,3,7)Try pushing up the VAS reservation by using the -g option.Also for linked server , plan to run out of process

4)Tune the applications to minimise VAS pressure - work with the vendor , who may have knowledge

5)Use sys.dm_exec_connections and report on network packet size greater than 4096 - Further details below

6)Consider FORCED parametisation - this will disguise the problem , consider returning to developers for development using parameterized queries

Make the relevant change - monitor the server performance for a few days and check if the issue appears again

Query to see if you have an application that is connecting with a network packet size of 8KB which requires multi-page allocations from VAS. 

If you do, change the connection string to include (Packet Size=4096)

select s.session_id, s.host_name, s.host_process_id, s.program_name, c.net_packet_size

from sys.dm_exec_connections c

join sys.dm_exec_sessions s on c.session_id = s.session_id

where net_packet_size >= 8000

Ref:Jack Vamvas (http://www.sqlserver-dba.com)