My two cents,I am not an "expert" ,so take my thoughts lightly. Anyway this is what I think. I would say Scar,s last statement is what is doing this. Just think ,a very wide wing so close to the stabilizer,it seems to me at high speed would create a lot of vacuum on the upper side of stabilizer,thus making it dive,more so if the stabilized is a "lifting type". Cure would be to raise the stabilizer above the wing or make a longer fuselage. It looks to me this model is not meant to go fast,but slow and easy and good for stunting,etc. Keep in mind "air" is a substance and as such acts much as water does on an object,only at a much higher speed. Example,, say you are going 1000 mph in a jet plane,if you could indeed stick your head out into the wind,what do you think would happen to it? I suppose that it is hard to think that air would be so violent,cause we live in it all the time and can't see it per say.
If this model was to be into a wind tunnel and tested at higher speed [where you could add smoke to show effects] I think you would see that that is what is going on[lifting stabilizer at higher speeds]. Well,guys and gals out there,what do you think?