I know everyone is going to immediately invite you to Audacity 2.1.3, so I will too. I don’t expect this to solve the problem, but support for earlier versions starts falling apart.
http://www.audacityteam.org/download/mac/
You can get chopping like this if your machine goes into App Nap.
Here’s a message thread we did on that.
https://forum.audacityteam.org/t/skipping/45947/7
Koz