Is the GTX680MX in the late 2012 iMacs OK?

I recently bought a new iMac with the upgrade to the GTX680MX and both FCPX and Motion are crashing MacOS entirely frequently (no error message, the whole OS stops responding and then a moment later, the mouse pointer stops responding and it remains entirely locked for at least 30mins - I've not waited longer). The machine passes the Apple HW test and everything else runs without crashing. I thought it could have been the thunderbolt drive I had the projects on which were initially creased on a Mac Mini, but it even happens with very simple brand new files/projects created in Motion - I think reproducibly. The exact same files/projects work fine on the Mac Mini.

Any clues? Have I bought a turkey since Apple don't mention the GTX680MX at all - I assumed as an Apple upgrade to the latest machines running the latest version of the OS and FCPX, there's no way it would not be supported