An overcrowded node (and here I am using the network engineer's generic definition of node, not Comcast's limited and redefined definition for a fiber-coax media converter), can happen with any shared network. An extremely overcrowded access concentrator was one of the reasons I dropped AT&T DSL.
What you are seeing used to be quite common on Comcast. But for the most part it has been fixed by their DOCSIS 3 upgrades (which is not to say that some areas might not still have that problem). You might try to get Comcast to re-provision your account and make sure that you have the proper config file pushed to your modem to see if that helps.
On thing you might try is to clone the MAC address of one of your PCs into the WAN interface of your Juniper firewall (and then reboot both the modem and the firewall). That might get you a new DHCP assigned address in a different IP address block than you currently have; and that might mean different routing and better performance. I have seen that improve things on Comcast connections (but it is a long shot).--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.