Talk about a strange problem. Today, everything got really slow. Called up Comcast and they pointed out a nifty tool called the Speakeasy - Speed Test. You plug a machine directly into their cable modem and it measure the real speed from a server. Here in Seattle, I got pretty close to the theoretical maximum (I've always thought that I was on a lonely segment and this confirms it). Got 5.1 MBps (wow!) and 355Kbps upload which compares with the 6MBps and the 384Kbps that they advertise.
Went a step farther and tried it via my D-link 624 router. Everything hanging but lights were OK. When I tried to use the web interface, the box was looking for a firmware reload! Wow, must have been too tough on the little guy. Anyway, fortunately had version 270 on a machine and the firmware reload took. As an aside, interesting to see the overhead that the router box adds. Got 4.4Mbps and 36Kbps upload, so there is definitely overhead in the router and then sometimes got 1Mbps and about the same for the upload, so tells me that all is not well with the router. I then started turning off PCs and it looks like I have the dreaded too many TCP/IP connections and this slows the router down.
Taking out all the machines and starting again, I got back to 7Mbps and 354Kbps, so the diagnostic is to:
# Direct connect one machine directly to the cable modem
# Run the Speakeasy speedtest
# Connect the router
# Check to make sure the web interface and the router is OK
# Run Speakeasy speed test again
# Reboot machines behind the router until you find the one that is slowing things down