I really don't think the C.G. is the main culprit. Check wing incidence, but I suspect it may be the incidence of the horizontal stabilizer. The leading edge sounds as if it may be positive to the thrust line. This would have the same effect as inputing some down elevator.
The following is from the Seamaster Assembly Manual. If you proceeded as outlined, the horizontal stab should be O.K. If not, it must be rectified.
"Mark the center on the bottom of the stabilizer at both the leading edge and trailing edge. Turn the fuselage over and block up the wing saddle so the top of the fin will rest on the bottom of the stabilizer. Make sure the wing saddle remains parallel with your work surface. The trailing edge of the fin should line up with the trailing edge of the stabilizer (hinge lines). Making sure the stabilizer is in proper alignment (use your center marks plus a triangle to make sure it is perpendicular to the fin), use a pencil to mark where the fin meets the stabilizer. Being very careful NOT to cut into the balsa wood, score and remove the covering material 1/16" inside your marks. Epoxy the stabilizer to the fin, maintaining proper alignment."

You can access the manual with this link:
http://www.modelflight.com.au/manuals/seamaster.pdf

Bob