I solved my problem. I apologize, but I neglected to mention that I had two 10/100 switches between my router and Xbox - I thought they were innocuous. My network configuration hasn't changed in many years, but I've upgraded both my PC and Xbox several times, so that lead to believe it was more a software issue. Anyway, all I had to do to fix my problem was unplug my PC from my first switch and plug it directly into the router. I now upload at a consistent rate of 10MB/s.
Let me explain my setup in case someone else has the same issues. I have one 4-port switch that connects to my router and another one next to my entertainment system that piggybacks off it.
ROUTER > 4-PORT SWITCH #1 > 4-PORT SWITCH #2 > XBOX
I unplugged my PC from switch #1 and plugged it directly into an open port on the router. So now my PC connects to the Xbox through the router. Strangely, if I were to bypass router #1 altogether, my upload speed drops by about half (maybe a nice networking guru can explain to me why that is). What perplexes me is that this seemed to transpire after I installed a bigger HDD in my Xbox.
In case you're wondering why I have two switches, well, at one time I had several people living with me.
Thanks for all your suggestions. I should've done more troubleshooting on my own before posting, but hopefully this thread will help someone else.
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