I think that you probably know more about this than I do.For USB, as far as I understand it, it needs USB interrupt mode which means that the system has to poll the TV card. If this polling is not fast enough, data is dropped because an overrunning buffer inside the TV card.
I am sorry that I have not been able to be of much help.
I hope that you can find a solution.
-- from CyberSimian in the UK