Let's not forget that an important reason why your download speed varies over time is the multifactorial 'general internet condition'. Personally (~350KBps DSL), I've seen the (almost) immediate maxing out and then petering off phenomenon too, but more often I see the opposite, with download speed ramping up until finished or maxed out.
Occasionally, a particular download settles at a certain, sub-max speed. When that's really slow (or when I'm impatient), I might stop the download and resume it immediately. Quite frequently, things then take off and quickly hit max speed until finished. Doesn't always work, though, presumably reflecting source server issues. Whatever is going on, certain trends tend to hang around for days or weeks before settling on another pattern.
