Its the Fixed background causing your stuttering when scrolling. Along with the Iframe too. Ive had the same problem, so i dont use IFrames that much anymore. Too plagued with browser compatibility problems to bother, who wants to make two website just so people on other browser can see ur work?
I suggest to go to frames, and slice the background up, so in the menu section u have one peice of the background and in another frame another peice and so on. Like a puzzle.
I see what your trying to do, there are better ways. You could make the entire site a table, and have the table have a trasparent background. This way you can divide the site in as many sections as you want, and not have to worry about the background getting screwed up. A table without a specified background color, takes the background of the page.
it will work with tables man, might save you a headache or two as well, and u can still use an IFRAME in the cells so ur forums can appear wherever you want.