Ok, so the internet at my parents is pretty good speed. Speedtest shows it to be between 25-30mbps, and while I know Speedtest is mainly e-peen, it's a pretty good test to show how well your internet is actually working. I have been using wireless internet for years with no problem at my apartment and at various other places.

Well, I set up an Asus RT-N12 router to their modem and speeds plummeted. 0.10-0.20mbps download speed according to Speedtest. Tried my computer in the other room with an Asus PCE-N15 and got the speeds. Hooked up my laptop right next to it, same speeds. Iphone, same speeds. Called Comcast, they said it was a router issue. Called Asus, they told me to change two settings (change channel to 7 and from 20hz to 40hz) but it did nothing. Then they told me it was defective. Took it back to the store, replaced it for another. Same deal, right out of the box.

I've updated firmware, reset the router, nothing seems to work. I'm being throttled constantly. Does anyone have any idea why this is happening and how to actually fix it?

The first test is to get a direct connection to the network. Rule out any interference. Are you able to get an ethernet wire between you and the router?

Yes, works 100%

Excellent, that mean's they're doing their job. Have you tried all the channels? It's possible that a local <provider> has been interfering with a frequency. If so, get the device as close as possible to the router, then slowly move away. See what stops it.

If you have an Android grab this, it'll tell you the best channel to use:
https://play.google.com/store/apps/details?id=com.farproc.wifi.analyzer

If you dont have an android you can sniff out the best channel with any laptop with this tool: http://www.metageek.net/products/inssider/

shoot for B or G mode it has the greatest range, double that of N mode... this one is very important. As you distance yourself the single and speed can drop.. you want a signal less than -75