Servo moves on its own when Motors running

Building an obstacle avoiding robot. When using the full code the servo was found to move in instances when it shouldn't. Even when removing large parts of the code, including all the servo orders as shown below, the servo continued to move in a sort of jittery uncontrolled manner. Here is the code below.
We are using a continuous rotation servo, ROBOT SERVO HSR- 1425CR.
Can anybody explain why?
Any help greatly appreciated!!

#include <Servo.h>

int motor_pin1 = 4;
int motor_pin2 = 5;
int motor_pin3 = 6;
int motor_pin4 = 7;
int servopin = 8;
int sensorpin = 0;
int dist = 0;
int leftdist = 0;
int rightdist = 0;
int object = 500;

Servo myservo;

void setup ()
{
pinMode(motor_pin1,OUTPUT);
pinMode(motor_pin2,OUTPUT);
pinMode(motor_pin3,OUTPUT);
pinMode(motor_pin4,OUTPUT);
myservo.attach(servopin);
}
void loop()
{
forward();
myservo.write(100);
delay(1000000);

}

void forward() { // use combination which works for you
digitalWrite(motor_pin1,HIGH);
digitalWrite(motor_pin2,LOW);
digitalWrite(motor_pin3,HIGH);
digitalWrite(motor_pin4,LOW);
return;
}