Probably isnt comcasts fault...
I've had comcast for 4 years now, and its one heck of a lot better and faster than DSL...
Heres what you do...
Use your comcast cable modem for the modem, and just get a linksys router... I find using the defualt equipment that comcast gives you, is usually better than what you can pick up at the store...
So Cable modem, connects to router.
In your router settings, what linksys is 192.168.1.1 in the internet address bar?
You basically set your connection type to automatic, you put in comcast.net for host name and provider name.
Then you clone the mac address (in the linksys settings).
The way comcast works is, the first computer you plug into the cable modem, gets registered to THAT modem, AKA if you unplugged your desktop from the modem and plugged in your laptop, it would not let you connect because it was registered to your desktop IP...
If you unplug your router for 5 minutes, and plug it into any pc, it will register to that pc.
My point being... Register the modem to a PC first, then clone the computer (that was registered to the modem) mac address into the linksys router. Or simply clone the modems mac which is now the same as the registered computer into the router...
Once youve done that, it should all be set up properly and you shouldnt have any issues.
The issues you are having are probably due to some of your equipment... Easy way to find out, spend some time hooking up different equipment seeing if you get the same response.
In some cases comcast has to come out and clean your cables or re-lay some new cable because your old cable was ruined by rain or what not. Usually it just wont work in that case...
Also have them check the cable filters or rather splitters on your home. Sometimes if you have a cable running from a pole to multiple tvs around your house, Electricians will install little SPLITTER boxes, that split the CABLE signal and INTERNET SIGNAL and allow it to run to mutiple places in your home...
These splitters have different frequincies and support different speeds... If the impropper splitter is installed, your internet can lag, work sometimes then stop, or just be rediculously slow sometimes and faster other times...
Moral of story is, get Comcast to put your splitters in, and not electricians... Ive been down that road myself.