Morton revs higher, speed down

[Eyeyiyi]

18’ pontoon Bf50a Honda after pulling it out and rebuilding carbs and doing fluid changes and new water pump I just got her back in the water motor ran great revs to 5500 at 16 mph. I bought the boat used and it used to go faster . The prop is originally and beat up so I got another alum prop same pitch 11.4x12 installed I only get 5000 rm but gained 2 mph which find unusual but acceptable. My problem is with the new prop after 5 to 10 min at wot all of a sudden the revs jump and boat slows. Checked to see if hub spun nope marks still aligned . Is there something else that I didn’t get aligned right when I did the water pump ?