The problem turned out to be with bad interaction with the network adapter and some of its driver settings. This NIC has settings for "Energy Efficient Ethernet" and "Green Ethernet". Both are enabled by default. I've learned those types of settings are notorious for causing problems. Hibernation and sleep modes, for example, also tend to cause problems. I think these two NIC settings were causing the adapter to partially power down when it shouldn't have, causing an 80-90% reduction in my upload speed. I disabled the two "features", and that seems to have done the trick. Whew! Days I've spend trying to figure out this problem.
Now, I have to finish up transferring software and hooking up my peripherals, and I'll be good to go.
Update May 30: Darn it all! I ran into problems again last night. This is very frustrating. Back to the drawing board AGAIN.