I think what you are experiancing is normal as your headspeed builds rapidly and the T/R doesn't have enough speed to counteract the rapid tq increase caused by the main blades. All helicopters do this, even collective pitch ones. But it is less previlant in cp heli's because the tail rotor runs at a near constant speed.

Lift your heli into about a 3 foot hover and use trim to counter act it - if the heli will hold a relitivly stable hover than there is no problem with your setup.