has it always been that way? those are good motors but almost everything gets wore out
before you get too much into it see if your compression is good and even across all four cylinders, you can rebuild that motor and make it a higher horse power for a lot less than a new 150 would cost, and still have a better motor, i am pretty sure you can make that motor at least a 130hp
your prop could be wore out! what size prop do you have, maybe we could do a prop swap test run, i have the same motor(just a couple years older) i would like to try a few different props to see if i can find one to perform on my boat a little bit better