Thanks for the reply.
I'm still doing tests but will try what you said
I've narrowed it down to a layer.
If I turn this layer off it doesn't seem to happen.
Soooooo, I then split the layer up into 2 parts:
PART 1 = FP & RC
PART 2 = other objects
I render them both (PART1 on & PART2 off / PART1 off & PART2 on
The glitch still happens
So either an object in both layers is causing this orrrrrr
Well, I don't bloody know :D
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The scene is quite heavy so I'll try and figure the cause before sending it. I'll post if I figure it out