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Servo choppy [Hextronik HXT-900]

I have three servos: hxt-900, parallax 180 deg, and parallax [continous rotation] from a bobot. When I run the example servo [sweep] in arduino, both the hxt and parallax 180 deg run very choppy; it's almost if it's struggling just to sweep the full 180. I've tried adjusting the time in ms as well, but it doesn't seem to have much effect. Only the parallax 360 runs without any problems and it's smooth as butter. I've tried two different hxt's, and they both have the same performance. What could possibly be wrong? I'm planning to use the hxt for my ping sensor. 


Thanks guys!