Pages show as the mobile version one day and the next not?

It sounds as if you have a caching plugin serving cached mobile pages to your desktop (we see W3 Total Cache in your source). You may configure your caching plugin not to cache mobile pages nor to serve cached pages when mobile devices are detected. To do so, enter the user agents found under Advanced Options > Custom User-Agents to the “rejected user agents” areas in your caching plugin’s settings. Remember to clear your plugin, browser, and device caches before retesting.