I've never had a cable modem, but from what I understand, this is common behavior among cable modems with default settings. While I don't recall the details offhand, I believe the resolution to this matter has to do with throttling back the upload speed cap on the cable modem. The reasoning for this was that the cable modem is being swamped with communications control packets when you are both downloading full tilt and uploading to capacity, overloading some buffer in the cable modem. Perhaps one of those computer-savvy persons to whom you refer would be better able to explain this than I, as I, to reiterate, have never had a cable modem. Anyway, look at your cable modem configuration and see if there is some way of artificially capping the upload speed and whether such cap can be lowered.