CAPTCHA cfimage tag providing unreadable letters

I'm using the CAPTCHA option in the cfimage tag to generate CAPTCHA challenges for web forms.  This is working but some of the letters are unreadable even on "low" difficulty setting.  The other strange thing is that this works fine on the development server but is working differently on the production server (they are both running CF Enterprise version 9,0,1,274733 on Linux).  Has anyone else run into this or have a solution?

Here is an example (again, this is "low" difficulty):

Thanks.