gilshultz:
Here are the specifications, Note the PWM range, It sounds like your PWM is to fast.
Those might be specifications for a servo but it's definitely not an LD-20MG which is a 20Kg-cm servo much larger and heavier than that.
OP -Trying send the power circuit to such large servos through a breadboard is likely to be a problem. Breadboard connections are fairly high resistance and can't handle high currents. Try making more direct contacts to the servo power leads.
What exactly is your "power cell"? Many AA batteries can't supply enough power for such servos either. Also the LD-20MG doesn't work well on anything less than 6V so 4 x AA cells are always going to be marginal. A 2S lipo would be much better.
Steve