I've had this problem before in the past 2 years. It will appear for a few days then magically disappear and not arise again for a month or two. Comcast troubleshoots over the phone, sends technician out, once they even sent a crew out to inspect the line. They never find anything wrong and problem will go away on its own.
Symptoms: Intermittent packet loss especially when playing online games. Ping will go from 40ms to 5000ms then back. Fluctuating speeds upload/download. Speedtest always shows slow upload, download varies.
Troubleshooting: Reset the modem. Refresh IP. Replaced modem.Use different computer. Replaced all splitters and connections in the home.
I'm going on day four now and still the problem persist. Tech came out and said, "everything checks out here" and left.