Been having the same problem, in the same June timeframe. I was sick (still am), so I was not working on my site at all. Went in and found much of the CSS not rendering in preview--suddenly. Sometimes it renders on the site proper, with a few exceptions. For a while I was seeing an error message that was wrong. I logged in two days later, and it had vanished. Wacky behavior when I hadn't changed anything. Same problem as in April.