The symptom is constant:
Reboot the machine, check broadband speed with speedtest and speakeasy: results 1300 up, 400 down, which is my advertised dsl speed.
After an hour repeat the tests: results 400 down, 50 up. This speed will only decrease until next reboot.
Tried so far:
Had dsl supplier verify that the trouble is not from their switch to my machine.
Wiped C drive, reinstalled XP home and updated to most recent that Microsoft thinks I need.
Ran Spybot and Super Anti Spyware. Theyre happy that nothiing's amiss.
Installed PCI network controller.
Ran tweaks at http://www.dslreports.com/tools. Changed settings as recommended with Dr. TCP.
Connected other computers to the modem. Symptom is not evident.
Task manager shows no unexpected processes, no memory leaks. Local processes run normally.
Have not swapped out the motherboard yet, as I'm trying to avoid that.