I'm having this same problem. Using a completely fresh .minecraft folder (I keep backups) and installing modloader, modloader mp, forge 1.33.12 and IC2 1.70b. I didn't use NEI to get the chainsaw or the circuit, battery or even the refined iron, but used it to get base materials to craft those myself. Using the chainsaw on logs and leaves works fine, but if I keep hitting the logs and/leaves after it runs out of energy it breaks.