Took it to an apple store. They ran a dianostic test on it, detemined that the nvidia chip on the logic board was defective. I was told that Nvidia had shipped a lot of defective chips and Apple was aware of it, along with other computer makers.
They replaced my logic board under warranty, even though it was over a year old and I did not have Apple Care. Great service!!!. When I looked at what the bill would have been - $1323.00 I couldn't be happier. Good thing tht the Tech guy at the store knew what was going on. He nailed it as soon as I described the problem. Hooked up a diagnotic device and found out the problem immediately.