Yosemite Grey Screen with 3d Party SSD not a trim issue I have a MacBook 7,1 which i recently installed a PNY 250 SSD and added some RAM. The laptop was running awesome with OSX 10.6.8, lightning fast. 

Then I installed Yosemite. There was a bit of hang during the install, but Yosemite came up no issue when I rebooted. Here is the problem:

Every other time I boot the MacBook it hangs at a grey screen with the prohibitory sign. Verbose reveals a "still waiting for root device" error. Now if I reboot from that hang, it boots fine, but then will hang again the next time. It keeps alternating like this. Any ideas??

I am not running any third party trim software, so that's not the issue.
try resetting your PRAM and SMC, this may resolve your issue, 

http://osxdaily.com/2010/11/15/reset-pram-mac/

http://osxdaily.com/2010/03/24/when-and-how-to-reset-your-mac-system-management-controller-smc/

otherwise it could be bad RAM... take your Macbook in to apple and have them test the RAM.