dippasan....
A boat porpoises due to overtrimming the drive relative to the speed the boat is traveling. Too slow + too much positive trim = porpoising.
Another cause for porpoising is a nasty hook in the hull. You can take a long straight edge to the bottom of the boat to see if this is the case. I doubt it though. If thats the case its not a cheap fix.
You didnt say who made the powertrain... I assume its OMC? In anycase I would follow the advice above and look at changing your prop. Not sure what the prop size was originally, (prob somewhere around a 14.25 x 21) but if you can find that out that would be a good place to start. Def go with a 3 blade stainless steel prop over the original aluminum. You can have some cup added to the prop to allow more bow lift.
Here is a link to Four Winns website...
http://www.fourwinns.com/past_produc...20(p12-16).pdf
Woodsy