Have you calibrated the ESC to the Arduino?
http://arduino.cc/en/Tutorial/Sweep#.UxHysGePKUk
The PWM range for the Sevo library is from 0 to 179. Your ESC probably doesn't know where the Neutral and FWD/REV start, with respect to the dutycycle coming out of the arduino. That can cause all sorts of problems. Write a function, or program that will calibrate the ESC.
What values are you using in your variables?
Shawn.