Entities and drops disappear or are unrendering

During gameplay, mobs (both hostile and not) and entities like boats and dropped items are becoming unrendered. At random intervals, mobs will become invisible yet will still be attacking and causing damage; also at random intervals, items dropped from mining will not appear until around two minutes after they're mined. Common thread is that all entities are doing this at random intervals, and they cannot be interacted with until they become visible again (mobs can't be damaged, items can't be picked up).