I have a Neptune (Seamaster knock off) and it behaved just like this. I found that I could eliminate it by mixing elevator with throttle with the computer radio CAP7 Futaba. Do you have a computer radio? I also added flaperons and find they a really nice for making very steep aproaches at slow airspeeds. I can descend at 45 degrees, power at idle, pullout at the last second, retract flaps and land in 15 ft. The water really slows the airplane fast.
Offgridman