Dreamweaver 4, mac os 9

I have been maintaining a website for 6 years.
All of a sudden I have been having problems with previewing my pages in the browser. I can open a dreamweaver document and the pages preview just fine. But when I close down the page and just click on the file to highlight and then select preview the navbar and the headers all come up as missing. I have always done it this way to be able to check just one file. Why all of a sudden does it not want to work this way.

Any help would be very much appreciated.
Thanks

Not all browsers behave the same. I myself have been experience problem, most common is mozilla firefox and IE, i have display that shows up 5px difference between headers and contents and when i open it using IE it goes all weird and the whole layout breaks up. May some of the display, shows somewhere else. Then on mac its worst my boss uses safari and we have built a website using only css. And guest what, its all messed up. I guess that the best way is using different browsers and build it up using IE (most common browser) then if preview is different in other browsers tweak until you find the exact cross over between them.