Specific Permalinks Give 404 Error

Using the (new) std %postname% permalink setting.

I’m running into an issue with a site I’m trying to update. It seems like certain specific permalinks in posts will give a 404 error.

At first I thought it was a -length- issue, but that’s not it. There’s something about certain ‘words’ that just don’t want to work. And if I address the shortlink that fails as well.

Ex:
http://suntowersystems.com/recalculate-usage — fails
http://suntowersystems.com/?p=3656 — fails

http://suntowersystems.com/calculate-usage-test — works fine
http://suntowersystems.com/?p=3656 — works fine

How to troubleshoot?

TIA,

—JC