hyperlink sometimes does not work

Sometimes a hyperlink on a page does not work (http://dbwork.com/werkgever/). It is the button ‘Werknemer zoeken’. Also at the same time not all fields in the form can be entered. A F5 does not help. Only entering the page again. It only happens like 1 out of 10 times entering the page.
As it is a published site I cannot deactivate the plugins. However I installed all the same plugins on another site, where I cannot reproduce the problem.
Does anybody else have this problem?