Mike
I have contacted Kona about this problem and I will let you know.
Please note that the footage 1st has to be re-sized and then graded and the the 10 bit sequence rendered 10-bit high precision YUVbefore you see the plaid like artifacts in the highlights. It is only displayed on the analog + SDI outputs on grade 1 and client monitor and not the apple cinema display in the FCP's canvas.
Also the problem still ocurs using Kona 3 version 3.3 drivers so it looks like a compatibility issue with FCP 5.1.2 and Kona 3 Version 3.3.
Cheers.
Malcolm