There really isn't much more I can describe besides my fresh Juno install is freezing when the system Idles for around 15 minutes or longer. I had a stable loki install before doing a clean wipe and re install when Juno dropped. I've got an rx480 gpu but am using the integrated drivers. Not sure where to start...
I had the same issue. It seems the system tries to go into suspend at around 15-20 minutes, but fails, and keyboard and mouse are non-responsive. Power button will not wake up. Wake on LAN will not work. Hard reset is required.
For now I disabled suspend. Must be done in the terminal as per this post: elementary OS Juno beta goes to sleep when it is not supposed to