I have manually set the timings and voltage. Loosening the timings and upping the voltage won't even make it stable at 1333mhz. I guess it's a motherboard incompatibility. They're sticks of CT25664BA133 not a kit, but the problem is exactly the same with one stick. It seems rock solid at 1066mhz so I may just leave it; the ram was only £28 a stick and I can always upgrade to 1600mhz or 1800mhz ddr3 when it gets cheaper.
edit: Next time I'll check the QVL before buying.