Networking → Uneven cable modem speed btwn 2 router/ntwk comps

Ok, here's my setup. I have 2 computers networked to a Linksys 4-port router by 50' CAT-5 cables to a Surfboard cable modem. For the first couple days, everything was fine, great download speed, surfing speed, everything. Then all of a sudden one of the computers started averaging 10k-15k/sec on downloads, and everything else slowed down similarly. The other computer is just fine still. This is the problem. Here's what I've tried, and if it worked or not:

1) Tried "slow" cable on "fast" computer - computer still had regular speed.
2) Tried "fast" cable on "slow" computer - computer started going fast again.
3) Thinking interference in the line somewhere, I tried "slow" cable and computer near the modem - worked fine. Tried it later a few feet away, and it was slow.
4) Thinking interference somewhere on the line, I brought the modem and router closer to the slow computer. The "fast" cable ran along the same path as the "slow" one did, and was still fast. The "slow" one was still slow.
5) At this point, I'm thinking problem with the "slow" cable, so I returned it and got a new one. I ran the line, no crimping on it anywhere (I checked all 50' of it visually) - still slow.
6) I also tried switching the ports the computers were connected to on the router - didn't do anything.

I'm out of ideas. I could re-install Windows, but I don't think that's the problem - the computer runs fine in some instances, remember. I'd move the computer, except in the only feasible locations, it's still slow. Aside from the ol' "I must be living in a magical anti-technology vortex" excuse, I can't think of anything that could be wrong. Both comps are running Win98, and the Control Panel\Network settings are identical. I could really use some help with this one.

Forgive the length of my message, but I thought the more information I gave the better. Thank you for reading.

Have you been to the tweaks forum and tweaked both computers for broadband?

Just a thought , if you think you have a problem with noise, at 50' of patch cable this could be part of the problem. try running cat 5 cable ( not a patch cable) and terminate both ends and use a short (3-5 ft) patch cable this should help if it is a noise problem