Look on your ISP's website for anything relevant, could be in Terms of Service, or Fair Use Policy, etc.
Another thing to try is basic trouble-shooting just to eliminate other potential problems: temporarily turn off firewall, antivirus, etc.
Could be also that your PC is too slow to manage the fast connection. In which case, try to find an optimum speed. But as I said, without more info about what's actually happening on your end, it's all shots in the dark at this point.