Intermittent performance problems

Hi,

I've been having intermittent performance problems where every few days download throughput drops from the plan rate of 100Mbps (sometimes even as high as 117Mbps) to about 33Mbps as measured by speedtest.xfinity.com from a system adjacent to the modem (Arris SBG8300). Other systems on the same network see a performance drop as well. The modem does not report any problems, nor does Xfinity, but a reboot fixes it for a few days.

Is there anyway to distinguish between a problem I'm responsible for (modem, in-house cabling, etc.) and something Xfinity is responsible for?

Thanks,

Skylar