I've had my fair share of stability issues with my 15" MacBook Pro (MBP) since I bought it in late 2009. I've had to replace the RAM, the hard drive, and dealt with all sorts of issues. I recently went on vacation and opted to leave the ol' MBP at home so that it, too, could have a vacation. I even shut it down all the way (how considerate of me!). When I returned home it fired right up, wanted to install a couple patches, and all that good stuff. And then it all started going bad...
After being home a couple days, my laptop started crashing with kernel panics (I call this the "grey screen of death" - OS X says you've crashed and need to hold down the power button to reboot). After a couple days of this nonsense, with it getting progressively worse, I decided to call Apple Support, which, btw, ROCKS. They had me shutdown, do an SMC Reset, then boot into Safe Boot mode (hold down shift before the boot-up chime) to remap drive and directory info. Viola! Problem solved! (or not)
Friday evening, after watching some Netflix, I closed the lid on my laptop and set it aside. About 10 minutes later I realized I had meant to look something up, so grabbed it and re-opened the lid. There I was greeted by a slow-pulsing blue screen. No login, no screen saver, no nothing. Just blue. What to do?
First I held the power button down and rebooted. No joy.
Then I shut it down and rebooted into Diagnostic mode (hold D before the boot-up chime). Hardware was ok. No joy.
Then I tried a SMC Reset. No joy.
Then I tried booting into Safe Boot mode. No joy. (same blue screen)
Then I tried a PRAM/NVRAM reset. No joy.
Then I booted to the OS X DVD (for discs, hold down C before the chime, or Option to pick your boot device) and ran "Repair Disk" under Disk Utility. No errors.
Then I ran "Repair Permissions" under Disk Utility. It corrected some things. I rebooted. No joy.
(I was later told by a tech that running "Repair Permissions" from the OS X installer isn't always a good idea)
Then I rain System Restore from my Time Machine backup. Rebooted. I'm in! Except...
When I logged into my system, I was prompted to allow or deny external access from mDNSResponder, cupsd and cups-lpd, and configd. I wasn't hosting any printers, so chose to deny the CUPS-related services. Not knowing of a reason to allow an external connection for DNS, and not knowing what configd needed info for, I denied those, too. BIG MISTAKE!
I couldn't get the wifi to work. It turns out that I shot myself in the foot by denying access for mDNSResponder and configd (especially the latter). Apple Support wasn't really sure why I was even being prompted to allow them as they shouldn't be denied if networking is enabled with DHCP. What do I know? Apparently I'm a little too paranoid for my own good...
The good news is that I did everything right. Well, except for the firewall permissions (how very un-Apple-like behavior!). Apple Support was awesome as usual! Once wifi was working again everything seemed to be back to normal. After letting it run ok for the rest of the day, I then opted to upgrade to OS X 10.7 "Lion" as I was feeling lucky. :)