Internet Download – “Throttle Down”!

I live out in the country side and my only choice for high speed Internet access is Satellite. DSL is not available. Thus, I currently have a DirecWay Satellite system. The upload speeds are horrible? somewhere between 2Kbs to 20Kbs! The download speeds can be awesome?. 400Kbs! However, on almost all downloads, it starts out fast and then slowly (sometime quickly) it ?throttles down? to 2Kbs to 10Kbs! Why does it do this!? It drives me nuts. It does the same thing whether I use a download manager or not! Please help me understand what?s going on. Thanks, Mark