Testing to see if mine shows up as unknown.

Nope.
Also running Windows 8.1.
Seeing as your guys IP is showing up as unknown, it's likely that it's grouping your IP's together causing the post withing 20 second flag to pop up commonly.
For example, let's say User A and B are both having the issue.
User A makes a post at 12:00:16.
User B attempts to post at 12:00:26, but gets flagged since User A is showing up as Unknown and the last post time for Unknown was at "12:00:16".
Edit B:
I cannot see why the forums cannot detect your IP though, makes no sense at all.