How are you loading the tab content? Is it in the page when it's downloaded, or are they loaded via Ajax?
If already present in the document, you should try hiding (display: none) the content div's - that way you won't see them all if the tab initialization fails (which is likely what is happening intermittently).
This sounds like it could be a timing issue - where are the tabs initialized? Make sure you have them in the document.ready function so you are sure jQuery and jQuery UI are fully loaded first.
After a failure you could try manually initializing the tabs from the console - the result (success or failure) should give some additional insight.
Also use Firebug to examine the content divs to see if they are getting changed by the tab initialization - there should be classes added.