stop pulling the trigger very slightly!
adjusting the "deadband" on your radio to a smaller one might help.
you did recalibrate the esc for the new radio, right?
the cheap radio just sent more pulses to the reciever with less input than needed with the new radio, but with all the adjustability of the futaba you should be able to gain similar performance.
if adjusting the deadband doesn't help, it's possible trimming the throttle a little forward could help, but too much and the esc will always see partial throttle and probably fry.
time to read the manual!