The reason for the twitching is simply the way interrupts work. When an int occurs, it 'bites' away time from the main code which is executing. In your case, this main code happens to be doing the pulsout function. The int could be disturbing this by an amount equal to the executing time of the interrupt routine.

The way around it - have a variable that tells the int how much pulsewidth you need to be sent to the servo. then in the int, which is always running, you send this pulsewidth to the servo. This way you will avoid the pulsout. Now, there will be no twitching

Hope this helps