Best to start by considering the partial power loss as an imminent total power loss.
So point the airplane at the nearest field and perform a cause check to see if power can be restored.
Assuming you have enough power power to maintain level flight then you can consider going for the nearest airport but arranging your flight path to always have a field within easy gliding distance.
If you can't maintain altitude then treat the partial failure as a total failure and close the throttle after manoevering to a good position to start your forced approach