Really does look like you covered most of the bases people would find a problem.
Ruled out the cables outside and inside.
Ruled out viruses with the format and reinstall
I know it isn't XP because that is what i still have as well and i run 30/5 just fine
2GB of ram is plenty to run XP
Does the system have on-board LAN, or is it a card? You may want to think about getting a card if you have an open slot. They are a world cheaper than building a new PC. And if you did build a new PC you might need one anyway.
Hopefully someone can come up with a few other idea's. What boggles me is that you said after they replaced the cable the old system DID get the speed it should for 2 days.
Thanks Steimy for your response. Yours is the only response I got, so I have nothing else to compare with.
The issue is still there, now the phone (connected to the modem, apparently a digital phone RCN calls it) is having issues.
They are planning a major overhaul, re-cabling from the drop-box outside the house.