They say that GD longtails are designed for a 16" transom (that is the measurment at the back of the boat from the top to the bottom). Alot of boats have 21" transoms and if you run a longtail on one of these the angle of the shaft and the water surface is wrong which causes it to want to rise out of the water. I think GD sells something to convert the motor to a 21" system ( I don't know that for sure). On the motor bracket right behind the motor there is a number cut (don't know how to spell ectched) in the frame Call GD give them this number and they will tell you anything you want know about the motor and where to find any parts you need.
It could be that the load is wrong in the boat. Try shifting the weight foward or backwards (probally with a 14 ft boat it is already back heavy). I have a longtail and it does better when the boat is running pretty level.
You can adjust the cavatation plate located right above the prop by bending the back of the plate up and the front of the plate down see this thread
viewtopic.php?f=6&t=906If it only does it when running wide open try running at about half throtle. On this setup there is probally only about 2 or 3 miles per hour difference between full and half.
If you get tried of messing with it, drag it over to my house and leave it. I would not mind having a setup like that.