funny apache performance on ADSL

I have a ADSL<->Linksys<->linux setup. The linksys has port 80 forwarding to
expose an apache on the linux box to the internet. This all works fine and my
relatives can download the small home movies I make.

I have noticed one peculiar behavior. When a someone starts downloading a movie
file, they get about 15KBytes/sec. This seems reasonable with ADSL, since I
understand the upload is much slower that download. However, about 3-4 minutes
into this activity, the rate seems to redline at 100KB/sec. I can see this as
the DSL modem light goes from low-duty-cycle-blinking to solid-on. The movie
download completes very quickly at that point.

I am not complaining, just trying to understand what is happening and if I can
get it to happen faster that waiting 3-4 minutes.

Phil