thanks for the reply – here is what I have
I have an image of a side of a building in a precomp – which is 2d
then I take 4 of those and a roof in another pre comp to make a building – this is 3d and all layers are 3d. (I actually have 6 different versions of this for different buildings)
then I have another pre comp with a grid of the 3d buildings – all layers are 3d
then I have the final comp which has the grid above as well as ground layers (with caustics) – a lot of lights – particular lux (for volumetric lighting)- some video clips placed within the grid
the only 2d layers that I have in the final comp are – a wave map (which isnt visible but used for the caustics) – the particular lux and particular partical effects – tried turning those off to see if that made a difference and it didnt.
if there is anymore info you could use to help id happily give it. thanks again