Firefox Radio Button Weirdness

I have a form with a lot of groups of radios. Each radio has a unique id and has the same name as the others in its group. The page validates as XHTML transitional. Tested in IE6 & 7, Opera, Safari, and Chrome it works exactly as you would think it would from either mouse or keyboard input. In FireFox it goes crazy. A single click on any radio in a group sets the first radio in the group checked. A double click on a radio usually selects it. Anyone seen this before? Sample group looks like this: ``` <input type="radio" name="upAndDown_1" id="upAndDown_11" value="Y" /> Y <br /> <input type="radio" name="upAndDown_1" id="upAndDown_12" value="N" checked="checked" /> N<br /> <input type="radio" name="upAndDown_1" id="upAndDown_13" value="NA" /> NA ``` The phonmomenon can be tested here: http://www.nolaflash.com/stackoverflow/firefox_and_radios.html Any advice appreciated.