"Re: 2006 Mac Pros w/Quad-Core CPUs, OS X Lion and Kernel Panics
My 2006 Mac Pro (upgraded to 8 cores with 2 Xeon 5355's) doesn't experience any KP's with Lion if I remove the lower memory riser. I noticed it was reporting that the 2GB DIMMs were only 1GB. I tried every combination of RAM possible to make sure it wasn't defective RAM.
(The comment on incorrect reported ram mix reminded me of an older article here - Mac Pro FB-Dimm Pairing Guide/Tests w/mixed 1GB, 2GB and 4GB pairs - however his ram config was ok until recently.)

Now I have 6GB of RAM instead of 12GB, (Riser removed) but I don't get any KP's. Maybe it's just a coincidence and my memory riser went bad right when I installed Lion. It would be helpful to see if it helps anyone else out.
Thanks, Darryl"

I wrote earlier today with some questions I had:

1. Were the KP's so frequent (before removal) that you could not run apple Hardware Test to see if it flags that riser/RAM?
   
   
2. Do you have another Drive with a pre-Lion OS to boot from as a test?
   (to see if riser/ram installed causes problems in another OS.)

Update - email reply @2AM Sat.

"The KP's occur when the CPUs are stressed like opening multiple applications and rendering video at the same time.
I was able to run memtest under snow leopard and apple hardware test booted off a CD and it didn't KP or catch any hardware problems.

I tried multiple memory configurations swapping different sticks in the lower riser and I can get it to crash every time, so I'm pretty positive either all of my memory is good or they are all bad. I don't get any KP's with the lower riser empty so that means the memory sticks are good.
- Darryl"

Do you have any util like Temperature Monitor to check what the FBDimm temps are during stress tests? (the FBDimms have a serial/parallel chip that runs at appx 4GHz IIRC and can get very hot - but properly heatsinked/good modules have been reported OK at over 80C in some tests.) I just wonder if for whatever reason, the lower riser area is running hotter maybe. But I dont know why Lion would have affected that. (Unless something is corrupted on the Lion install... I'd be tempted to do the usual voodoo drill (reset smc/nvram/corrupted file check, reinstall D/L updater, etc etc) - but just guessing. Makes no sense to me why lion would cause this assuming a good install/good hardware otherwise. But it's 3AM so I'm calling it quits for tonight.)

If it were me, I'd rather run 10.6.x with all RAM installed/everything working fine.

Update - email @3AM Sat.

"Something else interesting, I installed all 4 of my 2GB sticks in the upper riser (with the lower riser empty) and I can make a KP. Swapping out a pair of 2GB for a pair of 1GB (going from 8GB down to 6GB), the system is stable.
You're right, it is late. I will do more testing under SL with Temp Monitor over the weekend. Let me know if anyone has reported anything about removing the lower memory riser.
- Darryl"

So then it's not just an issue with the lower riser, but related to total ram installed? (addressing?)

If anyone else with a FBDimm (2006-2008) Mac Pro has seen anything like this, let me know. Thanks. (Will write previous readers w/MP 1,1/QC/Lion KPs, including those that updated to 2,1 bootrom.)

  

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