High Memory Usage

Have you investigated why the engines would be busy during the interval of time when the messages were kicked off and when the log entries are seen? You could then check the All Process report, and focus on those times to see what is occurring process-wise at around the same time.

If high memory usage (from your title line above) is the main concern, then you can review this KB article for ideas on how to track it down and try to resolve it:

forum.appian.com/.../kb-1248.html

Here is the correct link 

https://community.appian.com/support/w/kb/463/kb-1248-how-to-address-high-memory-usage-in-on-premise-appian-environments

Here is the correct link 

https://community.appian.com/support/w/kb/463/kb-1248-how-to-address-high-memory-usage-in-on-premise-appian-environments