cutsom posts 404ing

I can’t stop my custom post types from sporadically 404ing.

It doesn’t always happen and I can’t predictably replicate the problem – as in sometimes it happens, sometimes it doesn’t. However, once I clear the cache, the 404 always goes away and the URL works.

I have tried resetting and re-saving the permalinks, which are currently category/postname.

I have also tried playing around with the flush rewrite rules, setting them to blank and false, but nothing seems to cure the issue.

Here is my current code:

add_action('init', 'create_raceresults_post_type'); 

function create_raceresults_post_type() { 
register_post_type('raceresults', array( 
    'labels' => array( 
        'name' => 'Race Results', 
        'singular_name' => 'Race Results', 
        'add_new' => 'Add New Race Result', 
    'add_new_item' => 'Add New Race Result', 
    'edit' => 'Edit Race Result', 
        'edit_item' => 'Edit Race Result', 
        'new_item' => 'New Race Result', 
        'view_item' => 'View Race Result', 
        'search_items' => 'Search Race Result', 
        'not_found' => 'No Race Result found', 
        'not_found_in_trash' => 'No Race Result found in Trash' 
    ), 
    'public' => true, 
'exclude_from_search'=> false, 
'rewrite' => array('slug' => 'best-running-training-program', 'with_front' => true), 
'supports' => array( 
        'title', 
        'excerpt', 
        'title', 
        'excerpt', 
    'editor', 
    'custom-fields', 
    'revisions', 
    'thumbnail', 
    'author', 
    'page-attributes' 

    ), 
    'taxonomies' => array('category', 'post_tag') // this is IMPORTANT 
)); 
flush_rewrite_rules(); 
} 

Is there something I am missing or not trying? Here is an example of that custom post: http://runnersconnect.net/best-running-training-program/race-results-march-24-march-25/. I just tried it and it worked when I first published 60 minutes ago but just 404ed when I tried to check it.

Thanks in advance!