Mike & Nick,
This seems to be a persistent mac issue, I run a Late 2015 5K iMac with 32Gb Ram and I run into this issue as well. It seems to me that Dreamweaver on the Mac has a memory leak issue particularly when using Server Behaviours, this is certainly the case with CS6 and it would seem that it has not been completely solved even in the latest version.
I run a local server via MAMP that I do all my development in and it still happens regularly to me. Like both of you command + s has become my default after making any changes.
Now there are at least 3 decent people in the world !