Okay, I went back outside and found which cable was the right cable. It worked, albeit miserably, through the coupler and splitter, but as soon as I got rid of the splitter, it just didn't work at all. Then I got rid of the coupler and plugged the input cable directly into the splitter. That didn't really seem to help. Then I moved the output cable from the -3.5dB output to one of the -7dB outputs, and my packet loss seems to have gone away. The ping times jitter quite a bit, but throughputs are back to about 75% of what I got on Thursday. Certainly far more usable than it's been in 24 hours.

Does it make sense that the signal from Time Warner needs to be attenuated, or am I drawing an invalid conclusion?