Why would the center of a Prune Cake collaspe at the end of cooking?
could be physics, could be chemistry, could be both. everything from altitude to humidity to old baking powder to uneven oven temperature can cause a cake to collapse.
can you provide more information about the recipe so we can help you troubleshoot? and have you checked your oven temperature with a thermometer to confirm that it's calibrated properly and heating evenly?