I've had several of the Office Star branded chairs over the years. As another big fellow (height and weight) I have found every one of them eventually failed. It usually starts with the pneumatic cylinder leaking as they aren't rated for more than 200 lbs.

I eventually bought a Hon 7800 Series chair (http://www.hon.com/Products/Chairs/7808.aspx) and haven't had any issues at all for the last 3 - 4 years.

I easily spent more for all of the Office Star chairs that I discarded than the one Hon 7800 Series.