[Sephiroth]

Have you tried modem mode?

If that doesn't work, the usual reason is that whereas previously you were on an underutilised legacy network (as in channels and VM servers), when you were upgraded, you were put onto the DOCSIS 3 network shared with a much larger number of people. Although you have more downstream channels than before, it is possible, nay likely, that the ratio of users to capacity on the DOCSIS 3 network is higher than on the legacy network.