How's the dust and Heat inside your MacPro?
Have you removed the side door with the power on and checked that the fans are spinning on those cards?
Your Mac Pro has Error Correcting Code (ECC) Memory. Simple memory errors are corrected on-the-fly, and uncorrectable errors cause a Hard Stop with a kernel panic.
So do not spend a moment worrying about memory errors causing wierd things to happen in a Mac Pro.
The inside of my MacPro (and all my computers) is very clean - I keep a clean work environment and regularly take off the side and carefully blow out any dust.
All my fans (inc the two GPU's) are spinning correctly. I use SMC Fan Control* to manually control my fans to keep the MacPro cooler. It makes it a bit noisier, but my experiences with high power PCs is the cooler they run, the better. The air from the back of the MacPro is barely warm owing to the higher flow-rate.
As for the RAM, I didn't think that would be the cause, but again experience says sometimes just reseating cards/GPU's/PCI cards etc can clear up some issues - not in this case.
Thanks for the sugestions, but unfortunately, I'm still no closer to solving the issues.
Any more ideas from anyone?
* The program won't prevent fans from spinning at a higher RPM (if the SMC decised they need to), it only increases their lower RPM limit above 'default' so there's no risk of the machine overheating due to wrong usage of the settings.
Grant - I've only got the one extra GPU installed - the rest of the machine is stock standard. Apple support the configuration I have and would also 'allow' and extra PCIe RADI card if I'd wanted it. I would be very surprised if it was a power issue given the intent/nature of the chassis underlying the MP. As well, the machine has worked fine since I bought it and the issue I have is only weeks old.
X423424X - I'll try your idea as the GPUs 'know' when there's a screen attached (OS X will update screen layouts etc based on what's plugged in). It will be a pain to use only one screen though as I run VirtualBox on one screen and use the real estate on both the others. Still, might shed some light on the issue. I hope.