I have reformatted your code and the problem becomes very clear (to me).
Serial.print("Enter Position = ");
You are getting some input from the user and then setting the servo to that point.
When there is no data in the Serial buffer you set the servo to the value of write and delay 15ms before checking the buffer again.
I think what you want to do is if there is no data then you don't want to change the Servo position. In short move the
servo1.write(num); line inside the } of the while loop.
Now much more importantly the first time you enter loop() you won't have any data there so the servo will be moved to the number held in
num which is? Undefined, it could be anything at all from 0 to 4294967295 and if its the upper end of that it will take a while for the servo to get there, it might seem like all it does is spin. ALWAYS INITIALIES VARIABLES BEFORE USE :)
A minor point, your while loop could be, probably should be an if statement. That will prevent a cat sitting on the keyboard and blocking the servo movement.