Thanks Sue.
That link lead me to more troubleshooting tips.
Unfortunately, we still haven't found the culprit(s).

Here are some of the links we have come across.
How to measure the SQL Server MTL Memory
Troubleshooting memory leaks
There is no “MemToLeave” for the 64bit version of the SQL Server Engine!
MTL Consumers

This last link provided some new insight, but again no solution was found. Here are the results.
A : Although there are 3 linked servers, Only one is used and only monthly for a DBA task. 
B : Our XML serializing and deserializing are handle by in-house windows services outside of SQL.
Ci : No mention of 3rd party extended SPs in the sql error logs. No occurrences of "dll" or "XSP" in the error logs.
Cii : Ole Automation Procedures is disabled in sys.configuration. There are no calls to sp_OA stored procedures in sys.dm_exec_procs_stats.
D :  There are instances of multi page query plans, but they only add up to ~620 MB.
E : The CLR memory clerk is only using 30 MB.
F : We do not use MAXTRANSFERSIZE when backing up.
G :  There were connections that use a TCP packet size larger than 8192 KB. These were all SSIS connection managers and have been reset to 0.

Strangely the SQL server is still running smoothly. There are no memory related errors. The only symptoms are from the original issue of a SORT operator in SSIS hanging and another SSIS package that has crashed a couple of times out of its 1000+ executions.  

Any new suggestions are welcome.