Trouble with prop saver (motor cuts out)

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=4852&Product_Name=TowerPro_BM2408-21T_/_18A_BEC_/_1047_Prop_Combo

The speed controller seems to cut power to the motor at high 
RPMs. This seems to occur when my prop is slightly out of 
balance. I can get a little more than half throttle, and if 
I increase it, the motor cuts out completely. I am guessing 
that putting the prop saver on the shaft makes it even more 
difficult to get the prop to balance.

What are the solutions to this sort of problem?