Internet keeps timing out on me

My internet is sputtering, losing connection every 20 seconds or so and then immediately reconnecting. Here's the pertinent info...

ISP: Cox Communications
OS: Windows 7 Home
Modem: Motorola Surfboard SB5120
Router: D-Link DIR-655 R

Home network is set up as follows:

Cable > Modem > Router > Ethernet-connected Ooma / Roku3 + wirelessly-connected laptop

Here are the signal levels for my modem:

Downstream Value
Frequency 819000000 Hz
Signal to Noise Ratio 33 dB
QAM QAM256
Network Access Control Object ON
Power Level -6 dBmV
------------------------------------
Upstream Value
Channel ID 5
Frequency 35300000 Hz
Ranging Service ID 818
Symbol Rate 5.120 Msym/s
Power Level 39 dBmV

And here are the results of prompting tracert google.com:

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Clik here to view.

Any ideas?