I suspect you are using some process which is using nay extended procedures or clr ones causing memory leak
or
Have a external backup utility like veritas or litespeed purging memory
or
in any case set the max and min memory (remember both) to same value so that trimming is restricted completely for windows, and also enable lock pages in memory option in case its not already done
Prakash Heda
Lead DBA Team - www.sqlfeatures.com
Video sessions on Performance Tuning and SQL 2012 HA