The Servo library jitter is because it uses interrupts and the timer0 interrupt will
interfere with the precise timing.
If you are prepared to lose delay(), millis() and micros() then this line will turn off
timer0 interrupts:
TIMSK0 = 0 ;
If that doesn't cure the jitter you might have some interference issues (not impossible
with high current motors nearby).
The PWM outputs are hardware and thus immune from jitter, but only the timer1
output is 16 bit so that only pins 9 and 10 can provide accurate enough timing,
and you have to reprogram the timer hardware to get the right sort of frequency.