iMac picture pixilated and freezes up on start up.

When I start my iMac it seems to start ok at first, then the screen gets pixilated and freezes up. Some times I can get further in the start up process than others. I did just update to OS 10.10.3.

Mac (21.5-inch, Late 2009)

3.3GHz Intel Core 2 Duo

8GH Memory

Processor Name: I

Number of Processors: 1

Total Number of Cores: 2

L2 Cache: 6 MB

Memory: 8 GB

Bus Speed: 1.33 GHz