In this case we would suggest you to check the cable signal level of the modem to isolate the issue since you mentioned that you can see packet loss happening this will help us to isolate the issue. 

http://arris.force.com/consumers/articles/General_FAQs/S33-Cable-Signal-Levels

downstream power: 0 - 2 dBmV (QAM256 and OFDM PLC)
downstream SNR/MER: 40-41 (QAM256 and ODFM PLC)
upstream power: 39.5 - 41.3 dBmV (5 channels)

upstream power seems out of range.

I have upstream/downstream images but don't know how to post them here.

As you mentioned, Upstream channel power levels are not in a range. I suggest contacting your internet service provider to adjust the cable signals within a range or reprovision the modem.

xfinity adjusted the levels to my s33v2, which are now:

downstream power: -10 to -2 dBmV (QAM256 and OFDM PLC)
downstream SNR/MER: 38 to 40 (QAM256 and ODFM PLC)
upstream power: 49.0 to 50.8 dBmV (5 channels)

so they are all in range now. I'll monitor for a few days to see if this helps packet loss.

Even though the levels have been adjusted to the required range, the packet loss issue continues. Since the levels were updated there have been 11 periods of packet loss over 5%, with a peak of 43%. These periods last from 15 minutes to 3 hours, and happen at different times of the day. Xfinity is going to lend me one of their modems to compare losses, but at this point my S33 is suspect. It is only 8 months old, but I can find little information on how to get a replacement under warranty and no information about who to contact.

Regarding warranty replacement, I suggest visiting www.arris.com/consumersupport for further assistance.

Perfect, thanks for the link!

After talking to the warranty/tech-support department twice on the phone, it seems to be their position that if the modem light turns on the device is working, so it isn't a warranty issue. Their claim is that a modem is such a simple device (yes, they said that) that it either entirely works (with no problems at all), or it doesn't work at all. This claim makes the device rather unique in my experience, the only devices I've ever worked with could have all kinds of partial and intermittent issues. My rebuttal that any device can have partial/intermittent issues didn't sway them. They admit to being confused why a modem with all signals in the proper range would occasionally suffer severe packet loss, but offered no explanation of how this might occur.

To recap how we got here:

• modem started having intermittent loss issues at my previous address (3 or 4 months ago)
• after Xfinity installed service at my new location the issues continued
• Xfinity came and gave me a temp line externally until a new one could be buried, but the problem didn't get better
• Xfinity came and replaced all internal wiring between their external wire and my modem, but the problem didn't get better
• After receiving advice on this forum, I asked Xfinity to come again because the power levels were out of range, which they did and fixed the levels, but the problem didn't get better
• I reported the result to this forum, and then called warranty/tech-support, which didn't resolve the problem (or provide any hope of resolution)

Perhaps they need to be reminded of the "free from defects” spirt of warranties in general. If I sound unhappy, there's a good reason for that. Xfinity did everything they could...

Sorry to follow up on myself, I missed one item in the recap. Before coming to adjust the signals, Xfinity came and buried a new cable to my house. So, from my modem out to the pole, all my cables are new. I also replaced the cat6 cable between my router and the modem, just to make sure that wasn't the issue on WAN packet loss logging. I also switched ports on the modem, gigabit and 2.5 have the same problem.