I suggest to do a completely clean driver install. You will first need to do a driver sweep / driver cleaning and then reinstall the latest driver. What os do you use?

Any way it may be the GTX 285 using the cuda 4.2 app causing instability, which you cannot avoid if you are using a 600 series card with a 200 series card. You could try removing the gtx 285, doing a clean reinstall of the drivers, and see if there are still any problems with only the 680 installed.

Let us know what you try and the results.