Disappearing links although rare may occur for one of the following reasons:

First, when users mouse over a highlighted keyword, the Kontera tag makes a call to our server where our algorithm is constantly analyzing your content for the highest paying and most relevant Keywords. In the time between each loading of your site, an ad campaign that was initially chosen for that keyword may have reached its daily budget or impressions quota.

A second reason why Kontera links might disappear is because the algorithm found a better keyword to link, that is; it finds a more relevant keyword or one that has a higher paying ad associated with it. 

If this is something that you've noticed recurringly on your site, you may use the "Help" tab in the Publisher Center to contact Kontera Support and we'll be happy to take a further look.