The DMX input jitter is caused by the DMX receiver chip.
Only Transceiver chips MAX1480BCPI are affected
A fast help is to change the following resistors on the CPU board :
grandMA full-size, light, replay unit : R24 from 220 to 470 Ohm (Pin15)
grandMA ultra light : R5 from 220 to 470 Ohm (Pin15 of receiver chip)
To remove this issue completely, the CPU board needs to be sent to MA Lighting for calibration