Can anyone help/explain this for me please?

I've had this happen a few times before and now I've just bought Morphing Clothes and seeing a lot more of it....

The preview in PP2012 looks fine, no clothing pokethrough, everything fitting perfectly. The Firefly render though has pokethrough that's just not there in the preview. I've seen this before in a few small areas, but using Morphing Clothes it's dire, half the clothing has disappeared into the body!

Checked the same render using Reality and the Lux Render version is fine with no pokethrough at all. So it's only Firefly. Can anyone tell me what's happening and how I can fix it? I've tried searching forums etc. but can only find suggestions about Raytracing or high displacement values causing apparent pokethrough and that doesn't seem to be the problem here.