I just setup a new portal running cart v3.87 here http://www9.smith-consulting.com and it filters products when the category menu is on the same page as buynow and also the cat menu is on the product details page (another page). I hide the container and the print icon and the cat menu still filters.  The cat menu works on www7.smith-consulting.com portal also.  Both are running cart v3.87.  In order to fix it we need to be able to replicate it.
 
 
    - Do you have any errors in your dnn event viewer.
 
    - Do you have the buynow page name selected in the category menu settings.
 
    - In product setup did you add a value for seo url.
 
    - In the category setup did you add a value for seo url
 
    - Did you upgrade the cart from a previous version
      
The following are a few things you can try that may fix it:
 
    - Make sure all the module settings are filled out properly
 
    - Clear your browser cache and temp files, etc
 
    - Remove the module and re add it to the page
 
    - Change to the default dnn skin that comes with dnn their might be a conflict in your skin
 
    - Reset iis
 
    - Download the latest cart v3.97 from the downloads page and copy all the dlls from the zip to your \bin folder
 
    - Try a clean install of the cart v3.97