dreamweaver course - Page & table layout & formatting

Hi, I've inherited the site which I now maintain & update. I'm finding it very difficult to control formatting - successive screens which should all have the same basic layout are now slightly different - most commonly the left margin moves around, shifting everything on screen slightly to the left or right or sometimes screen elements move up or down from one screen to the next. Looking at the design view, I can't see what the problem is - I've read through manuals & web guidance without success. I find the design view is quite unpredictable anyway - grateful for any hints for getting it under control.