mrspabs on "intermittent 404 errors in dashboard"

wordpress gives me 404 errors when i click on things in the dashboard. operations like upgrade wordpress, upgrade plugin, install plugin, deactivate plugin

i reset the permalinks to see if i can fix, but it doesn't help.
anyway, i get 404 error on pages like this one
http://site.com/wp-admin/plugin-install.php

when i refresh the page, the command sometimes goes through.

i suspect malware but the scans that I ran all came up clean. i have also uploaded a fresh copy of wordpress, upgraded all my plugins.

my .htaccess looks like this

RewriteOptions inherit

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

any ideas on how to fix? thanks!