1989 550SX porpoising at speed

I just bought a 1989 550SX, the exact same ski I had when I was back in England. The motor is strong, however the ski has a tendency to porpoise at speeds above half throttle. It accelerates strong, but then the bow starts rising and falling until it jumps out of the water and the impellar can suck no water. I've tried standing all the way to the front and hanging off the back (I'm a big boy at 6'3" 250lbs).
Any ideas would be appreciated. Thanks!