Is there any play on the wings, that might cause wing incidence change?, loose covering that might lift up when the wind pressure runs over it? even a minor thing can cause this behavior.

On another note, the Beaver has a very high lift wing, you'd better check the wing incidence and its relation with engine and stab, all that wing should be at around 1 to 1 1/2 deg. positive, as mentioned above, never more than 2, if you can't change anything at this time, you could increase its down thrust (engine) by another deg. or so.