I also get the selective rendering problem, mainly with the area render function. When it decides not to work right, it renders black. Sometimes a full size render will render half way and then leave the rest of the screen gray.

I've found that lowering the bucket size tends to help with this problem. On a big enough render I'll even set the bucket size 8 or 12.

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AMD FX-9590 4.7ghz 8-core, 32gb of RAM, Win7 64bit, nVidia GeForce GTX 760

PoserPro2012, Photoshop CS4 and Magix Music Maker

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...and when the day is dawning...I have to say goodbye...a last look back into...your broken eyes.