According to your TV's specs it only supports up to 1080i. (INPUT SIGNAL
TV/Video comp. 480i, 480p, 720p, 1080i) Which may explain the problem.
(I've always believed this but never had the equipment to test) I think that HDMI has only set resolutions that can be displayed . And most of these are based on the 16:9 format. (since HDMI and DVI-D define an overlapping minimum set of supported resolutions and framebuffer formats to ensure a basic level of interoperability : Wikipedia "HDMI")
So unless you want to use a VGA cable or switch out your TV I think you a little out of luck.