download speed tapers down to zero and stalls

I can connect to a server and initiate downloads. Sometimes I have no problem whatsoever. On other occasions I may run for hours or days, other times only minutes or seconds before my speed wil quickly start to decrease until it bottoms out at zero. Then I stay connected, but nothing is transferred. I have also experienced a roller coaster of inconsistent speed before it finally drops to zero.

What would cause this?

Most recently, I was able to stay connected for several hours and d/l 1 GB; upon reconnecting to the same server the next day, I could not maintain a connection, yet no settings changed on either side.

Thank you for any help or feedback.

Scott