servo sweep troubles

Hi I recently got an arduino and am trying to make a servo sweep back and forth without using the delay funtion. When I load my code, the servo sweeps back and forth several times after which it freaks out and starts flipping back and forth a small amount very quickly. I've tried rewriting the code a couple different ways to no avail. Here's my code, any help would be appreciated!

#include <Servo.h> 

Servo myservo;
int pos = 0;
int prevmillis = 0;
int interval = 15;
boolean growing = true;

void setup() 
{ 
  myservo.attach(9);
} 

void loop()
{
  if ( millis() - prevmillis > interval ) {
    myservo.write(pos);
    
    if ( growing ) {
      pos += 1;
    } else {
      pos -= 1;
    }
    
    if ( pos > 179 )
      growing = false;
    if ( pos < 1 )
      growing = true;
    
    prevmillis = millis();
  }
}