There's this option in page cache settings that looks related:
"Reject HEAD requests: Disable caching of HEAD HTTP requests
If disabled, HEAD requests can often be cached resulting in "empty pages" being returned for subsequent requests for a URL."
Unfortunately it's grayed out and unchecked for me, and blank pages happen often enough for readers to notify me a few times a week.
I also have had some timeout issue when posting and emptying spam recently, so I may have other issues as well.