You are asking a news group instead of an authority such as Lockwood, a
certified Rotax repair station? http://www.lockwood-aviation.com/

OK here goes:

A five (or ten!) year old 912 certainly should have been gone through
before installing it on your airframe. It will need new rubber seals on
the water pump as a minimum. The gear box on the 912 80 HP (black valve
covers) has been very good. Not true with the 912S 100 HP engine (with
the green valve cover). Rotax has added a slipper clutch and a higher
torque starter for the 912S. Your problem may be related to the prop.
Does it meet the inertia spec for the Rotax? Too high an inertia will
eat your gear box.

Have you read the installation manual at www.rotax-owner.com and
signed up for service bulletins?