Clipping sounds like a circuit stability issue to me - My carrier has a very nice tool, open to the public here:
It will give you a very nice estimation of your circuit, although it only tests against their POP’s in Chicago and LA - Still, it’s worth looking at. Needs Java.
Here is the list of what Comcast modems are compatible at what speeds and service types:
http://mydeviceinfo.comcast.net/
Make sure you pick the actual speed you are trying to achieve - the first Modem/Router I bought (SBG6850) is in the lowest tier of Comcast business so I was getting 30Mbit, but they were paying for 75Mbit and when you move the slider up, the modem drops off - thanks Comcast. It’s funny too because that modem will go all the way to the top of the residential offerings - wankers!
Since we have been converting most of our IT customers to Cloud-Based E-Mail and Collaboration solutions for years, Static IP’s are not nearly as important as they used to - and remember with a Modem, and decent router that supports Dynamic DNS, you can be found for any type of remote access - even better, make an alias record in your actual domain refer to the dynamic address you use and you don’t even have to make your people remember weird Dynamic DNS host names.