It sounds to me like too much collective. Your doing the classic..."Its dropping, add more power"...unfortunatly, at full throttle, the pitch is so high, its bogging down the headspeed, and it drops like a brick. This is a hard thing to recognize, because you know you've added power, and you can hear the blades beating the air frantically, so you would think it would pull out....but it doesnt.

Try adjusting your pitch curve to have less pitch and more headspeed at high throttle settings.
Headspeed is critical... too little and it will drop like a brick no matter how much pitch you put on the blades.

Ive tried to teach this principle to newbies. I unplug the motor, and tell them...ok TAKEOFF....
They increase power, but the heli just sits there. I explain to them that while you can see lots of pitch in the blades, without them spinning...it wont work.