Wow, very vauge explanation but my best guess would be:
1) Faulty Cable connection
2) Low Gain on your Phone Line (see **)
3) ISP has time limit (as per old Dial-Up service)
4) Firewall problem causing connection to terminate
5) Incorrect Settings e.g. MTU too high causing packets to constantly drop appearing as though internet is 'down'.
** Depending on your Phone Line you can ask your provider to "Increase the Gain on your Line" which should increase the reception and connection of your calls and broadband service. This usually works best if you are a long way from the Exchange.