iMac 27 late 2009 - Grey screen shows at startup on yosemite
I had the same Issue on my old iMac - it turned out to be defective Memory of the Graphics Card (due to overheating). 

I changed the Graphics Card and fixed the problem. There is a very well written tutorial on how to change the Card on iFixit. The only problem would be to find a replacement Card - i paid about 360$ for mine.