Are you a beginning pilot? I ask, because if you are, then negative pitch will just get in the way. So, when the throttle stick is all the way down, what pitch are the blades.

To find out, UNPLUG the motor, turn the heli on and check what pitch you have at the blades. I suspect that as you increase throttle, you are getting negative pitch. If thats the case:
1) blade grips installed on new spindle backwards. The control arm should be following the blades, not in front of the arc.
2) blades installed upside down (i did this just last weekend)
3) did you fiddle with the reversing switches by accident?