Why is my iMac not booting properly?

My mid 2011 iMac is having trouble booting. It chimes, gets around halfway through the loading bar, then cuts to black screen and begins the process again. Occasionally it will be boot to login screen but as soon as I put my password in and hit enter the computer blacks out and then reboots. I know there is a lot of trouble with graphics cards on this model. Is this a symptom of a bad graphics card?

It was the GPU. The T key on boot confirmed it. I tried the oven bake trick with my GPU and it fixed it! Booting cleanly now. Thanks guys!