What are they promising you? As in how many channels down and how many up? Normal is 3/1 or 4/1, or 3/4 channels down and one up. His issue may also be a CMTS, or atleast something inside their network, but its not effecting channel bonding, is just creating a large latency between gateway and CMTS. Probably a totally different cause, but probably the same type of issue.
I also had a problem with bonding with a 8/6 modem on a network that is set up for 3/1. Was never able to solve mine so I just went back to my 6120 but I think it has to do with legacy support for the older docsis 3.0 modem with the new docsis 3.0 modems. Just a total guess though.