I'm having this exact same issue, starting yesterday. Here are the signal levels from my modem.
Located in Phoenix, Arizona and I pay for 1gbps, started dropping to 32mbps yesterday randomly. I have a 4 year old Arris S33 modem so I don't mind trying a new one, but others have said new modems don't fix the problem.
What speed do you get direct to the S33? Remember you have to reboot the modem each time you change the ethernet. Also, what do you see in your logs around the time of the problem? Don't post your logs unless you edit out your MAC address. Your signal levels look good though and I doubt it's a hardware issue.
