Mosaico template editing not loading consistently
After using Mosaico from some time, I've noted that you need to make sure you've correctly installed this extension as per the steps mentioned in the doc, i.e,

Doc mentions you need to install shoreditch theme, flexmailer and then Mosaico in order.
Make sure you've downloaded all the dependencies it needs to be included by executing ./bin/setup.sh -D from your terminal.

After these steps, it worked fine for me on around 4-5 sites.