Hi. I have the same problem running Moodle 3.1 on Bluehost Cloud Hosting Performance plan (4GB, 4 cores). We had a surge on the user activity 4 days ago, so the server was struggling. While waiting for advice from bluehost support I tried some tweaks only within Moodle (i have no access to configuration files like php.ini).
One of the things i tried is running cron manually, since the the cronjob scheduler was not working. I'm not sure if this has anything to do with it, but since the broken images have a link like 
maybe one of the tasks did a cleanup on draft files.
As of the performance issues, Bluehost advice was to buy an additional 1 GB of RAM and 1 core. For the moment we bought the RAM, let's see how much it helps.
