I am using Display Suite to add a php button/link to the page for a single content-type. It works great. The title and the custom field (the button) appear as expected.
After some time, less than a day I believe, the title and button are no longer visible.
Checking the content type, the setup for the display has not changed. Clear Cache All fixes the problem...for a day or so.
I would like the Display Suite custom field and the Title to stay in place on the page.
Your case resembles the problem described in this drupal.org issue. One of the module developers thinks it is mostly related to out of memory exceptions.
A possible solution, described here, is to increase the following PHP settings: