ljbeng:
A Quick test... If I changed to hardware serial and 6s delay, no jitter. Is it tricky to have a permanent sensor connected to the hardware tx and program from the IDE at the same time? I may have a standby pin on the serial receive device I am sending to.
That suggests you should try the ServoTimer2 library in conjunction with SoftwareSerial.
I think you need to put resistors to prevent the permanently connected sensor from interfering with the other use of the Rx/Tx (as you can see I am not an electronics expert) and you probably need to ensure the sensor is off. Does the sensor need to be connected to the Arduino Tx?
...R