The connection was reset. (Why am I frequently getting this message?)

Dim 8400 hard-wired to US Robotics router #8001---RoadRunner broadband. Too frequently when I'm browsing, I get a window with this message:
------------------------------------------------------------------
The connection was reset

The connection to the server was reset while the page was loading.

*The site could be temporarily unavailable or too busy. Try again in a few
moments.

*If you are unable to load any pages, check your computer's network
connection.

*If your computer or network is protected by a firewall or proxy, make sure
that Firefox is permitted to access the Web.
-------------------------------------------------------------------

Occasionally, I can click the "Try Again" button on the window and it "may" connect to the website I was trying to reach. Sometimes, I can click the button several times and it just will not connect. Then, I can disconnect power from the router, wait a few seconds, restore power and all works well. Also, I can bypass the router---connect into the RR modem and, rarely, if ever have this problem. I have my WAN setup on Dynamic IP Address. Any ideas? Thanks!

Dimension 8400
1 GB SDRAM
3.4GHz
160GB
Samsung CD-R/RW
Philips DVD+RW
Iomega HDD 80GB USB 2.0
Radeon X300
WinXP w/Service Pack 2
USR 8001 router
RoadRunner
Firefox browser

Dimension E510,P4,3GHZ
512MB DDR SDRAM
128MB ATI Hyper Memory Radeon X300 SE
160GB ATA Hardrive
WinXP Media Center
48X CD-RW /DVD Combo
RoadRunner
Firefox browser