Hmmm, a friend recently had a similar problem with a new PC though in his case the modem seemed to loose power completely. It turned out that his motherboard had problems powering the modem. The solution? Go out and buy a four port high speed USB card.

Alternatively, if you don't like the idea of opening up your PC and inserting a new card (my friend happens to be very comfortable with this kind of work), I suspect a powered USB hub might also solve your problem.