When you see issues where VM's are using unusually high amounts of memory, you may be affected by the following bug where Rsyslogd uses excessive amounts of memory.
There is a memory leak in Rsyslogd 8.23.0 which is the version used in PCF1.8.21-build.1 in Stemcell 3263.12.
For example, check for errors like the following in your message logs (/var/log/messages):
Jan 3 16:39:24 localhost vmsvc[838]: [ warning] [guestinfo] Failed to get vmstats. Jan 3 16:39:24 localhost rsyslogd: action 'action 0' resumed (module 'builtin:omfwd') [v8.23.0 try http://www.rsyslog.com/e/2359 ]
You can check the version of rsyslogd that you are using by SSH'ing onto a VM and running the following command: rsyslogd -version
You will need to upgrade your Stemcell to version 3263.15 or later. In this version, the Ubuntu stemcells have been PIN'ed to rsyslog to 8.22.0.