I had exactly the same problem when I was upgraded to an 8Mb/s line (LLU in my case). It was definitely the modems inability to cope. Interestingly, when I used the modem with an older 1MHz PC it was okay but when attached to my 3.2GHz PC it crashed and required the reboot you mentioned.
My solution was to upgrade to a router, which connects via an ethernet port. This works fine and the download speed increased at a result as well.