Yep, sounds very much like the governor needs attention.
I've flown a similar age model U206, with a two blade prop, and at MAUW she climbed at about 800fpm, 25squared.
SL, about 15C.
As an aside, it could be damaging to the engine to run it at this low rpm and a comparatively high MP. Make sure you keep it fairly rich, maybe full rich, if you absolutely have to fly in this condition, as long as the MP is significantly above about 24" (if I remember correctly.These figures are guesses only.)
The POH (if present) should offer correct guidance as to permitted rpm/MP/mixture-fuel flow combinations.