Random freezing of MediaLab when using 'Responses.xls' file

Salutations!

When running a MediaLab study in which the Responses.xls excel file is used to compute values based on previous participant estimates, occasionally MediaLab will 'freeze': no question will pop up, no 'Continue' button will be available, and only the 'Scale responses' will appear.

For instance: participants provide two estimates of some value. Later, they are required to respond 'higher or lower' than a number computed by the Responses.xls file based on these two estimates. Intermittently, the question will not appear, continue will not appear, and all that will be on the screen will be '1: Higher, 2: Lower', but clicking 1 or 2 or with the mouse does not advance the study. Pressing Ctrl+(forward) still advances the study one slide. I am at a loss for why this is happening: it does not necessarily happen in the same place, on the same computer, or at the same duration of the study completed. Is this some issue with the Responses.xls file? The computers are using Windows 7 and MediaLab 2008. Do you have any advice?