Hello,
Usually a blank page is the sign of a PHP error somewhere. The only way to be sure is to display all errors. Unfortunately this is not very practical on a production site. Go to Administration ► Server ► Debugging and select Debug messages:ALL, then try to display your Questionnaires and see if an error message is displayed and report here.
However please note that it is always very difficult to diagnostic what you call "an intermitent problem". It's up to you to track the exact conditions in which the problem appears and the conditions in which it does not. This takes a lot of experimenting and a lot of time.
All the best,
Joseph