Have you tried the 811 Adjustment? Within setup, HDTV setup (Menu, 6,1,9) you can adjust the position of the picture. When you select 'adjust', you will be returned to the channel you were watching with instructions to move the picture. It moves in very small increments. When it is adjusted, select save.

What you are describing is a keystone picture, narrow at the top and wide at the bottom. You should be able to do some type of convergence/alignment calibration on your TV. This is not a typical and easy adjustment, that is to say advanced HDTV users could tackle this. Go to AVS forums and search for service menu adjustment in the RPTV forums. You could also search the groups.yahoo for a Mitsubishi HDTV group. They both should have procedures for getting into and playing with the service menu.

The 811 has no adjust features to compensate for what you describe.

If your Mits is still under warranty, I'd get a service call initiated to fix this problem. Yes, you can go into the service menu, but you can also make your TV unwatchable. Get some professional help.