I have a Speedy Bee by Clancy Aviation. It is a shoulder wing short coupled plane. The wing cord is very large and is a flat bottom design (some say semi semi semetrical).

When flying at very slow speeds the plane is pretty stable. However, when the speed is increased the plane will suddenly dive (Bee Tuck). The dive can be recovered with the elevator. This dive is unpredicatable and occurs without warning. Any theories on why this happens?