Hi Jack, I was having similar issues with my Mac with the identical OS. Here's what worked for me:
1. Change the Settings -> Energy Saver to about 2 hours
2. Use the Google Chrome browser. (Usually I use Safari)
3. Download PRE 2018
That worked for me and it might do the trick for you too.
Thanks - worked for me also from Google Chrome !!
I usually use Chrome and have had times where I had to use Edge. It does not always make sense.