Denny, you're exactly right. Sounds like a prop geometry problem, with possibly added CG issues.
First step, go to a 4 blade, but drop the pitch by at least an inch.
Propeller geometry can make a difference of more than 30% in your thrust efficiency. When I ran a V6 Merc on my Seafari, I switched from a 3 blade Mirage 19"P to a 4 blade PowerTech 16.5"P.
I went from:
5900 max rpm at 43-44 mph (30% slip) and 4000 rpm at 32 mph (26% slip), to
6100 max rpm at 52-53 mph (8% slip) and 4000 rpm at 34 mph (10% slip).
I don't know what the fuel economy was with the Mirage, but with the Powertech, I cruised at about 3100-3200 rpms and 24-25 mph from Key West to Marathon and got very close to 3 mpg with the V6 xR4.
Today, with my 90 hp Optimax, I can cruise at 4100 rpms at 24-25 mph and get 5.1 mpg.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury