Sorry, maybe it was the way I worded it! Are you experiencing judder when using apps on the Q box, like Netflix? Or is it judder when watching TV channels?I have a C8 and watch most content via Sky Q with no problems. The Sky Q box is connected to HDMI1 on the C8 via an HDMI cable (duh!). This must be what was meant by 'with Sky Q'. I can only think, assuming that both the Sky Q box and TV are okay, that it is the HDMI cable.
If it's when your using TV apps (on the Q box) there is a setting called judder reduction in settings - setup - audio visual.