Mac Pro 5,1 crashes/shuts off when starting to render video with OpenCL...

I would try rearranging the the cards in slot 2-4. Maybe a bad interaction

Above from:

http://old-macs.com/misc/MacProPCIePower.html

whose controller #3 for SSD? and do pull and just troubleshot and then decide, but cannot troubleshot if you won't try. Also boot8ing from another drive, invoking TRIM and yes all SSD need it or trimforce. an SSD can fail, and crashes require steps to insure directory and files are all ok - they won't if you just keep booting over again.

I understand that, on paper, your power draws all seem to work.

But your symptoms are exactly what Users report when they have installed a graphics card that overdraws the available power -- All works fine until they push the graphics card to its limits.

Perhaps the graphics card is close to or within its own limits, but you are overdrawing the total power when using the RAID card as well.

I would make certain the wires on the graphics card aux power connectors were suitably large, and that they were plugged in completely. (no marginal connections.) If one featured wires that look small by comparison, I would spend the US$10 to get a better-looking one.

Everything uses "wiping" contacts, so just removing and re-seating sometimes proves helpful in that it wipes away any accumulated corrosion.

and yes, the power supply is designed to shut down when over-drawn. The alternative is to allow it to run-away, overheat, and explode.

If one of the required two aux cables had wires that looked like the 18 gauge wire used for the other wires inside your Mac or car accessory wiring (which is perfectly fine), and the other had wires looked like telephone-wire (which is too small to carry the full current needed), I would replace the lighter-gauge one.

It's not likely, but neither is the problem you are having.

For testing purposes you obviously need the 7950 to see what your doing, and you need the Kingston to boot from, and you obviously need to have the ATTO card in as that is the suspect device. However you could remove the Sonnet USB card. At a minimum this would slightly lower the power load and also allow you to try the other slot for the ATTO card.

Have you verified the fan is working on the 7950? It might be worth giving the card a good clean as dust acts as an insulator and can contribute therefore to overheating. Does your 7950 look like this one? http://eshop.macsales.com/item/Sapphire/100352MAC/ That is does it have white plastic like that picture? If it has red plastic then it is a PC version not a Mac version. It is possible for PC versions to be 'over-clocked' which increases their performance but means they need more power and produce more heat. The Mac version is supposed to be clocked at 800MHz but some PC ones are over-clocked to 1GHz. I believe PC 1GHz versions are supposed to use a 6-pin plus an 8-pin cable whereas the Mac version uses two 6-pin cables. 6-pin cables only provide half the maximum power of an 8-pin cable so you can see a 1GHz card might draw more power than two 6-pin cables can provide.

I would agree with others that these symptoms are typical for a power overload. On the software side there is no way to get and install newer drivers for the 7950 since these drivers are provided by Apple and are built-in to OS X and you already have the current latest version of 10.10.5, but is there any update for Adobe Media Encoder?

John Lockwood-

Do you know if the clocking on this card can be controlled by a Utility? there were some older ones where you could fiddle around with speeding/slowing the clock, and a Utility to manipulate it.

I think all the tools for doing this are Windows only. On Windows these days you could use the Catalyst drivers from AMD which have built-in options to adjust the speed of the card. I believe there are some tools around to customise the firmware before you flash the card so as to set the default speed for the card and therefore set what will happen when using it in OS X as well. Obviously the Catalyst driver will not work in OS X.

This tool http://www.techpowerup.com/forums/threads/vbe7-vbios-editor-for-radeon-hd-7000-s eries-cards.189089/ looks the best Windows tool for editing the settings of a HD 7xxx series card. I think adjustments made this way will also apply to when you boot in to OS X.

ATI WinFlash is the tool for 'flashing' the firmware e.g. to convert to a Mac card - once you have made the Mac firmware for it.

Is your card a white plastic or red plastic one?