Comcast should be peering with other top tier ISP's besides Level3. Ormuco peers directly with Level3, TATA, Cogent, Verizon's UUNet Business class division, and TiNet. Depending on your region, they SHOULD be able to route you to at least one of the other 4 in that list.
They've been monkeying with their announcements for a few months now, so seeing a lot of outdated data on specific ASN's, but they do have peering set up with each one of those in their global peering list. Hard to say for certain exactly which of them would be available from your region without a specific IP to use to lookup the ASN for that particular network.
But... it should be possible to switch you to another peer. Rather, it IS possible for them to switch it...the questions are whether they have a peering arrangement with one of those other providers in your area, and whether someone will approve for them to tweak the policies to better optimize the routing for your gateway(s) there.
Edit:
yeah... found a list of addresses for some of their route servers:
Code:
68.86.80.0 - Ashburn, VA
68.86.80.2 - New York, NY
68.86.80.5 - Chicago, IL
68.86.80.6 - Chicago, IL 2
68.86.80.7 - Denver, CO
68.86.80.10 - San Jose, CA
68.86.80.11 - Los Angeles, CA
68.86.80.12 - Atlanta, GA
68.86.80.13 - Dallas, TX
Looking at the ID's assigned to them...they SHOULD have options available besides JUST Level3 if they aren't willing to lean on Level3 or otherwise improve QoS at the exchange point into Level3. (I pickup TATA in Ashburn, and when they put me on Cogent I go to Atlanta for the exchange--Comcast can do the same thing if you are close to those areas).
The trick is getting in touch with someone who can and will actually do something to help improve the routing in your area.
I know.. some of that data may make your head swim if you don't follow this stuff. The point is, they DO have options...but they are going to try to shank the responsibility and blame someone else. You may have to stick to your guns and push for supervisors or higher tiered support or something until you get someone who says yes. Sometimes they will do eveything they can to discourage people from pursuing it further...sometimes you have to go into bulldog mode (or raging chihuahua, whatever suits your style) and not take "no" for an answer.
Remember... you are paying them for internet service, and what you are getting is shoddy internet service within the scope of your usage. They don't advertise a quality service to just specific services, but internet service--that is a very broad network of networks, and the one they are assigning you as the go between to this particular section of the internet is not performing to a reasonable level of usage and THEY need to be held accountable to remedy it, as it is THEIR policy that is putting you on that troubled network. Not SE, not Microsoft, not Netflix... but COMCAST is deciding you should use Level3.