MacBook Pro loses access to 1 memory slot upon shutdown or reboot in OS X 10.8.1

i had (have) exactly the same problem, and the same computer (macbook pro 5,5). i'm convinced that its a design flaw with this logic board, and is something you just have to live with. what makes me think this?

1) i've only ever used 10.6 on this computer, so i know that the software upgrade is not the issue. it's happened on every sub-version of 10.6 with me. it 100% definitely for sure happens on SL. your noticing it on OS upgrade must have just been a coincidence.

2) i brought this computer to the apple store for apple-care-covered service when i first encountered it, which was in 2010. they were convinced it was ram, and so replaced the ram. problem persisted. they replaced the logic board. twice. problem persisted. i upgraded my ram with nice OWC ram, the problem still persists (BUT i will say that it happens less often now, not coincidentally related to the fact that the PCB on the new 4GB ram sticks were a fraction of a millimeter thicker than the old 2GB sticks). i know that the ram, and all the logic boards were really replaced because the ram had different serial numbers (reported in System Profiler), and the cpu id used for some copy protection changed with each new motherboard. further, i know it wasn't the ram, because even within only one set of individual logic boards/ram sticks, it was always the same slot that would go inactive, even if you swapped the sticks. again, this could be verified by serial number in system profiler to make sure they were actually switched.

3) when does the problem occur? often it will fail to wake from sleep. this is because half the ram disappeared, resulting in an incomplete loaded system, and thus failure. it fails to wake from hibernate in this situation as well, since the sleep image is twice the size of the hobbled ram. if you reboot verbose from this situation, you'll see the "sleep failure code 0x000015" (where i've surely gotten the number of 0's wrong, but you get the picture). for me, this seems to happen most often when either i carry the laptop around asleep and it gets a little jostled, or if i leave it asleep near a window, where the temperature can fluctuate quite a bit where i live. this point i see as directly related to the thickness of the ram sticks.

usually i can reslove the issue by shutting down again, and then rebooting, but there are times when the only fix is to open up the back and re-seat the ram sticks. needless to say, this is frustrating, and at first points to fautly (as in failing) hardware, but given what i've gone through with this, i think its just majorly inconvenient and not a sign of hardware failure.

except for this issue, my computer runs flawlessly. if you never rely on sleep, then it doesn't seem to be as much of a problem, at least for me, as i seem to always have full ram if i startup from a shut-down state. it seems unlikely to be a malfunction with this many different components. this points to design flaw.

i'm pretty sure the person on that other forum that was linked to wasn't lying, as the problem sounded all too familiar. that person reported that Bank1 was always the slot that went dead. that's also the slot that always went dead on mine. Bank0 always shows up.

(i think this issue might be underreported, as many users simply might not notice that their ram is missing.)