How to I diagnose downloads slowing over time?

I am trying to track down a strange networking problem.

Basically, a http download from a tomcat server is slowing down over time. For example, the download speed starts at 700kb, then slows to 650, 600, 550, 500, etc.

I have heard TCP/IP windowing might cause this problem. To make this problem a little stranger sFTP does not see the speed decrease.

I am confident it is not a problem with the webapp. It is running from tomcat and a Linux red hat machine.

Go easy on me, I am not a network guy.

GC_