From your description of it's movements, it sounds like it could be a buzzard. They tend to rise up on air thermals, circling as they ascend. Once they get to the height they need, they then just glide off, they do not seem to like wasting energy flapping!
We get a few where I live, they are beautiful, I was watching about 6 of them a few months back on a lovely blue summers day, riding the thermals.
They tend to get attacked by crows and other carrion eating birds, I guess the competition for food is quite fierce.