What pitch are you running? If you follow Finless Bob's recommendation, a 5 point pitch curve in normal mode works out to be 40,45,50,75,100. This works perfect for me, about -2 degrees at 0 throttle and +11 degrees at 100% throttle.

If it is not a pitch problem and you have tried (2) good Lipo packs, I guess it could be a defective ESC or motor. Do you have a spare to try?