This is the problem. I not infrequently come across this. I see a Kontera ContentLink, hover my mouse over it, and a blank "pop-up" or "tool-tip" or whatever you may call it, appear:

I watch the blank "whatever" for about a minute, then it disappear, leaving a normal word or phrase which used to be a ContentLink:

I will report this again to Kontera and see what is their response.
Update 17 January 2008: Response from Kontera: "we are aware of this issue but always appreciate getting feedback from our users.
The issue you're describing results from the efforts of our relevancy engine to display the best, most relevant ad per selected keyword. On the rare occasion that such an ad is not found, the link may disappear. This occurs quite rarely with ContentLinks and I'm surprised to hear that you have come across this occurrence several times. I assure you that our teams are working hard to resolve the issue."
Note from author: Let us hope Kontera resolve this problem soon.