ask Dr. LJ - broadband packet loss

Since about January, I've been noticing that my internet connection has been slow. I'm on a password protected WiFi network, so it couldn't be somebody stealing bandwidth (could it?). Last night I tried to download a 13MB file and it took most of an hour and sat at 6-8 Kbps for most of that time. I called RCN and they diagnosed packet loss, but could only tell me that it wasn't my cable modem and it wasn't a regional issue. So I messed around for a while, and eventually found that the "forward signal to noise ratio" is off. (It's too low; I was getting 30.3, and it's supposed to be over 32.) The question is, does the SNR cause the packet loss? And the other question is, what do I do about it?

I've inspected all the cables, indoors and out, and nothing looks wrong as far as I can tell (there's a bit of a rat's nest going on outdoors, but it's been like that for ages). I don't think I have any unused ports, and I have one splitter that goes to the cable modem and the cable box. There's nothing wrong with my computer, either. So, Dr. LJ, what the hell is going on here?