Powercycling the modem. I'll generally do the same to the router at the same time. As I said, "I've completely bypassed the router at this point..."
Advertised speed is "up to 5Mbps".
AntiVirus isn't the problem, as one computer uses McAfee, one is scanned regularly online using MicroTrend's Housecall (or was until speed bottomed out), and the third was just reformatted and has barebones software on it.
Only regular firewall used is anything that's built into the router, but since the router itself has been bypassed, that's a non-issue as well. Like I said, I'm fairly certain there's no problem with the computers themselves, because it's doubtful they'd all be experiencing the exact problems at the exact same time if the issue was with a comp.