Month view broken – intermittent

I’m using WordPress at http://leghumped.com/blog/, but I always give the address as http://leghumped.com/ on forums etc. I’ve done a bit of custom mod_rewriting to suit my needs, and sometimes I get a wordpress 404 when I try to view certain months’ archives, even if there are posts that month. It only happens on some months, not on others. My .htaccess is below:

RewriteEngine On
RewriteBase /
RewriteRule ^$  /blog/
RewriteRule ^feed/?$ /blog/feed/
RewriteRule ^/wp/([^/]*)$ /blog/$1/
RewriteRule ^index.php$  /blog/index.php
RewriteRule ^sitemap.xml.gz$  /blog/sitemap.xml.gz [NC]
RewriteRule ^sitemap.xml$  /blog/sitemap.xml [NC]
RewriteRule ^gallery([^/]*)/?$ /blog/gallery/ [NC]
RewriteRule ^archives([^/]*)/?$ /blog/archives/ [NC]
RewriteRule ^page/9400/?$ /blog/inspiron-9400/ [R,NC]
RewriteRule ^page/magician/?$ /blog/magician/ [R,NC]
RewriteRule ^page/magicianrom/?$ /blog/magician/ [R,NC]
RewriteRule ^blog/magicianrom/?$ /blog/magician/ [R,NC]
RewriteRule ^page/xp/?$ /blog/xp-setup-screens/ [R,NC]
RewriteRule ^page/([^/]*)/?$ /blog/$1 [R,NC]

To see what I’m talking about, http://leghumped.com/blog/2006/12/ doesn’t work, however http://leghumped.com/blog/2007/01/ does. You can see that both months have posts at http://leghumped.com/blog/archives/

Thanks for any help.