Prop RMPs ~2650 on Takeoff, but can't go above ~2500 in flight

[takair]

Based on Byron’s description I start to wonder if something is not right in the governor. It seems like something happens at the end of travel where it has a non linear response. I wonder if some break in debris may have gotten into the mechanism or internal valves and prevents it from going back to 2700 once it’s running. If I have my governor theory right, you are looking to reduce oil pressure to get flat pitch.  Maybe the “valve” is held open allowing oils pressure to regulate high. 
 

I will say, I had an issue years ago where FOD damaged my governor. In that case it would behave differently with different oil and even the age of the oil. 
 

Not sure how one would diagnose. Good prop shop can put it on the bench and test it.