Could be a number of things. Basically the RPM is not being controlled/managed.
This exert from a maintenance book I have........not sure if your prop was just overhauled. Look carefully at the last paragraph below.
One interesting problem that can happen at prop overhaul is improper pitch-stop settings. On a single-engine propeller (where oil pressure increases blade angle), a prop may be set up for the naturally aspirated model of the airplane instead of the turbocharged model. For instance, a C-T210 prop may get set up as a C-210, or an A36TC prop may get set up as an A36 prop. The props for a turbocharged aircraft model actually have a higher blade pitch stop setting than the prop of the naturally aspirated aircraft of the same basic model.
The symptom that shows up here is the inability to keep RPM down at a particular density altitude and airspeed. Higher altitude or higher airspeed does nothing but make the symptom worse; i.e., the higher or faster you go, the higher the RPM goes. And when you point the nose down for descent, the RPM really takes off. In essence, it's acting just like a fixed-pitch prop. Once you reach the altitude at which the symptom shows up, any further increase in altitude, airspeed or engine torque causes a corresponding increase in RPM. The prop is against the high pitch stop and can't go any higher.
On twin-engine aircraft (where oil pressure decreases blade angle), it's the low-pitch stop that will occasionally get set improperly. This will show up as the inability to reach proper static-RPM because the blades can't go to a low enough angle to allow the engine to spin up to max. RPM. Once you're rolling on takeoff, however, the RPM comes up, and for the rest of the flight everything is just fine.
A note of caution here: On the single or twin, this particular problem will only happen after prop-hub maintenance of some kind. A similar symptom showing up suddenly or gradually, with no maintenance having been done on the prop or engine, indicates a weakening engine or possible governor-system trouble.