Two techniques may help with this troublesome issue. First, try allowing your dough log to rest overnight in the refrigerator. This relaxes the gluten in the dough, making it more pliable. Wrap tightly before refrigerating, then bring to room temperature before slicing.
A tool that may help solve this problem is a sharp serrated knife. Rather than using a straight-blade knife, use a serrated blade to saw gently back and forth across the log. Do not press too hard—let the knife do most of the work and your biscotti should slice perfectly.