What is causing degrading connections vi

What could be causing my modem connection speeds to drop dramatically?
Bell Atlantic says nothing is wrong, but my modem speeds have dropped
dramatically recently. What kinds of tester should I borrow to check out the line losses, etc.? What values do I need to check? What connections among the various lines in the house could be suspect? I do have two telephone lines, one local, one FX (foreign exchange). The fx line offers slightly better connection speeds, but only marginally. Computer connection from the upstairs bedroom office is much faster than the main connection from the first floor office.