Rain can be another cause of disconnects. We have had users that had fire ants in the box where it comes into the house that caused disconnects.
Cordless phones cause disconnects.
Often the problem can be caused by the line going from the computer to the wall going bad. To debug this problem, start by disconnecting all devices from the line except the computer. Unplug the phone line from your computer and attach to a regular telephone. Dial the Internet access number and see if you get through using the regular telephone. You will hear the normal modem sounds if it is trying to connect.