Internet constantly cutting in and out, never achieving the speeds I pay for

What do the modem's signal stats look like ? Try getting them here http://192.168.100.1 or here http://10.0.0.1

Please post the *Downstream Power Level*, the *Upstream Power Level*, and the *SNR* (Signal to Noise Ratio) numbers.

What is the exact make and model number of the modem ?

Is this a WiFi connection ?

I can't log in there - each time I try, I get a prompt that says "incorrect user name" despite me copy+pasting it from my xfinity account. 

If that is a Comcast supplied gateway device, the default username is admin and the default password is password

Thanks, that did it. Here's what I've got:

Downstream -  

Power Level

Upstream - 

Power Level

SNR - 

The power levels and the SNR's are out of spec. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.

In a self troubleshooting effort to try to obtain better connectivity / more wiggle room, check to see if there are there any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage types like GE, RadioShack, RCA, Philips, Leviton, Magnavox, and Rocketfish from big box stores like Home Depot, Lowes, Target, Wal-Mart etc. Splitters should be swapped with known to be good / new ones to test

If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two way splitter connected directly off of the drop from the street/pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed, and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.

Thank you for the insights. There's only one splitter between the main line and the modem, but it's likely been there for decades and I wouldn't doubt it's low-quality. Do you have any reccomendations for good quaility splitters? I googled the specs you mentioned, but most of the options look about the quality of the brands you warned of. 

Be advised that it may or may not be that splitter, YMMV, but here are some quality brand splitters that Comcast actually uses in different market areas;

Antronix CMC2002H 2-Way Splitter: http://www.amazon.com/Antronix-CMC2002H-2-Way-Splitter/dp/B001E4OH1E

Regal 2 WAY 1 GHZ Splitter: http://www.amazon.com/DIG702867-Regal-WAY-GHZ-SPLITTER/dp/B0018BQMUM/ref=sr_1_2?s=electronics&ie=UTF...

Extreme Broadband BDS102H 2-Way Digital Coax Splitter: http://www.amazon.com/EXTREME-DIGITAL-PERFORMANCE-CABLE-SPLITTER/dp/B007YV0UQW

Holland 2-way horizontal splitter 5-1000 MHz with ground.http://www.3starinc.com/holland_2-way_horizontal_splitter_5-1000_mhz-w-ground.html

Sv-2g 2-way Splitter 5-1000mhz: http://www.amazon.com/Sv-2g-2-way-Splitter-5-1000mhz-Sv2g/dp/B003TH36CK

Thanks for your suggestions, I really appreciate your help. I'm sorry to be annoying, but do you have any suggestions for 3 or 4 way splitters, or advice on doubling up on them? I only need one line to the modem (obviously, lol) but there are two cable boxes in the house - would I be okay running the main line to one 2 way splitter, running one of those lines to the modem, then running the other to a second 2 way splitter that feeds into the cable boxes?

That's a typical configuration but be advised that the more cascaded splitters that you use, the weaker the signal will be. It's already weak so the signal strength to those cable boxes may be insufficient. Try it. YMMV.

Alright, I grabbed a couple of the Extreme splitters which are arriving Saturday. Despite the issue with the internet, the cable boxes have generally been fine, so I'll cross my fingers and hope this works. Thanks again for your help, I'll report back this weekend with the results!