OS X 10.10.3 boots into stalling grey screen

This issue is a recent one for me. It happened on April 28th. I powered on my iMac, and waited for it to boot. Entered my password to login, but then it stalls at a blank grey screen. I powered down my computer, and tried to login again, but the same occurs. So for the entire week, I've been trying to get my computer back to life. I've read a lot of forums for possible fixes but none of the fixes worked.

Here's what I've done:

-booted into recovery, repaired disk and permissions; restarted and still no go

-booted into safe mode, deleted unused programs, repaired disk/permissions again just to be sure; restarted and no go

-installed os x 10.10.3 onto an external hard drive, but the same occurred on that boot drive

-zeroed the external drive, installed snow leopard onto the external, then installed 10.10.3; still no go

-reset the PRAM; still no go

I am writing this post in safe mode on my iMac. I've been running Yosemite with no problem up to this point. I didn't see this issue coming, that's how smoothly my computer was running.

All diagnostics say everything is fine (hard drive, ram, graphics, etc).

my computer:

iMac 11,1 / 32gb ram / 2.8 GHz intel core i7 / ATI Radeon HD 4850 512mb

OWC 480gb Mercury 3G Electra SSD <-- boot drive running 10.10.3 (this is not the external drive)

Magic Mouse & wired keyboard

I'm running out of ideas on how to solve this problem. If anybody knows a possible solution or what could be causing it, please let me know. I've wiped my SSD (primary boot drive) and reinstalled the OS and still no luck.

Thanks!