Ralphh - those setting still show you at 50% precision of interpolation. In my post about the image that worked, I said that was turned off. I don't know if you tried that but do. The only thing is it actually slows down the render time to have the interpolation down or off. But in my case, it worked. Actually, I see now that you have it unchecked but I'm not convinced the % amount is still not being taken into consideration. That's why I also suggested have that % down to zero as well as the setting unchecked. I had caustics in another render still render though I had unchecked it. There was still a value (though greyed out) in the setting. So try turning the slider down to zero as well.

GOOD LUCK!
Dave