After working with Premiere Elements 2018 for a short time, I realized that the problem I had in the past with the timeline playback stopping "at random" was still a nuisance. Timeline playback would stop without any action from me anywhere from 1 second to 3 minutes after clicking the play button. The entire length was actually 90 minutes. To make a long story short, what finally fixed the problem was disconnecting the LAN connection going from my router to the computer.
Besides disconnecting the cable, there are other ways to disable the network connection. In Windows 10, there is now a convenient button to "disable network connection." See attached image at end.
I also found a tech site that describes 3 ways to disable a network connection: https://www.petri.com/3-ways-disable-network-connection-windows-10. I tried the first method, and that worked.
After disabling the network connection, timeline playback may stop within a short time the first few times you try, probably because of queued-up messages. If playback does stop, go to the Help menu and select "User Forum." You should get a message that there isn't a network connection. Then try timeline playback again.
The only time now where playback stops is when Premiere Elements does an auto save.
So if your timeline playback seemingly stops on its own, try disconnecting from the network. There's obviously some internet communication going on in another window that causes the Premiere Elements Editor to lose focus and stop playback.