Prop slipping?

I've got a Blazer 202 ProV with a Yamaha 225 and should be well into the 70's from what I hear. I'm running a 27p prop and the most I've got out of it is 66mph (I've had the boat 3 years now). I've tried raising the jack plate a bit but it didn't make much of a difference. When I get it on plane the RPMs seem normal but once it breaks over and I'm starting to get up to speed, I lose RPMs. If I back off a little and step on it again sometimes that will help. Other times it just randomly boosts and I have decent RPMs again. Could this be due to my prop slipping? And how can you tell when your prop is slipping? RPM fluctuates between about 4400 and 5300 when this goes on. I feel like I should be in the upper 5000's. it just haven't got there. Should I try a different pitch prop?

To give a proper answer, let's get some proper questions asked. Diagnosis over the internet is not often easy.

So, let's hear exactly what your rig does WFO...full power, full trim, what's the exact speed and rpms?
Holeshot speed?
At WFO, how is the lift? Where does the water break under the boat? How high is the roostertail? How stiff is the steering?
And the real question, 27 pitch WHAT? You might have a great prop, might have a turd, don't know.

I'll answer as best as I can.
Pretty sure I haven't got over 5500 RPM since I've had it
I'm not even sure what brand the prop is, it's something I haven't heard of so it may be a turn and it has a few very small dings
Lift is alright when it's running at the higher range of RPM
Rooster tail is usually right around the top of he motor. Sometimes a tad higher but not much more than about 7-8 feet
Steering is a tad stiff when on plane. The boat is a 2001 model but does have hydraulic steering

Only think written in the prop is 27-M

That's a Yamaha pro series. A good prop. But might be a tad stiff for a 225, but I'm still not clear on what it's actually doing. Can or have you held it down with full throttle and full trim?

That's the number that will lead you down the happy trail.