Mac Pro 2.66 crashing with more than 2GB ram

Question

I curently have a base config of 1GB ram (2 512MB chips), the systems runs fine, I then added memory in various configs and it always kernel panics when I exceed 2 GB. System runs fine until i try something memory intensive like Aperture or Photoshop.
tried
4 x 512MB chips (2GB) = perfect
6 X 512MB chips (3GB) = crash
2 x 512MB and 2 x 2GB (5GB) = crash
2 X 2GB (4GB) = crash
I have tried only riser 1, still crashes even with only the 4GB running. All memory is genuine apple and passes the hardtest CD.
Any help much appreciated
cheers

Answer 1

Agreed.
If you cannont get the memory to fail when only one or two pairs is in a given riser then it will be very difficult to diagnose and you'd have to suspect either a riser or mainboard problem.
You do know that memtest will only test up to 2GB unless you buy the $0.99 version 4.14 though right? Lower versions shouldn't KP though, should just allocate 2GB and test.
If it fails in both the following combinations:
Riser A - 2x 512
Riser B - 2x512, 2x 512
Riser A - 2x 512, 2x 512
Riser B - 2x512
then I suppose I'd be suspecting hardware failure on the motherboard. BY the way, both risers boards are identical, you can swap Riser A for Riser B and still have a working system if it helps in your tests.
I just went through the same thing, fortunately I had only 2 pairs (2x1GB) to work with...so I labelled the chips ABCD and worked the combinations through the risers A and B to determine that failure followed a given FBDIMM....after I'd done all this and found the suspected FBDIMM, I powered up the machine with the cover off for a change and found the red lights...did I feel silly then