iMac hanging on boot after 10.10.3

Greetings all,

After upgrading to 10.10.3, my iMac (12,1) started freezing regularly, necessitating a forced shutdown and restart. After two days of this, it failed to boot up at all, but would automatically power down about 1/3 of the way through the boot process.

I detached all devices except the keyboard and mouse and tried booting into Safe Mode with the same results (shut down). I then booted into Recovery Mode and ran Disk Repair. It found that there were errors with the disk (I did not make note of them at time), and that I should backup what I could (already done via Time Machine), reformat the HDD, and restore the backup.

I did this and everything seemed to work OK. However, upon returning home from work, I found that my iMac was frozen again (the mouse moved the cursor, but neither the Dock or any apps would respond). I tried rebooting again, only to find it hanging about halfway through the boot process.

I tried booting into Safe Mode again, but it would hang at the same point. I booted into Recovery Mode and ran checked the disk and everything is fine. So I then tried booting in Verbose mode, and got to this at the fsck_hfs point:

Executing fsck_hfs (version hfs-235)

Warning: couldn't block sleep during cache updae

Warning: proceeding w/o DiskArb

/dev/disk0s2 on / (hfs, local, journaled)

AppleAHCIDiskQueueManager: :setPowerState(alpha-numerical series, 3 -> 2) timed outafter 101096 ms

And that is where we are stuck.

Of course, this thing is, like, ONE MONTH past the AppleCare warranty's expiration date.

Any advice would be appreciated, or even any input as to whether the problem is with the system, or with the HDD. This is the first iMac I've had in since 2001 that hasn't last me six years or more, so I am kind of bummed... =(