One of two things: if it truly is random, you should check your system for
malware. Otherwise, check your connection settings. Start > Settings > Dial-
Up Networking > Properties. On the "Dialing" tab:

Never dial a connection. (Just as it says.)

Dial whenever a network connection is not present. (If you are not online,
you will get the connection pop-up.)

Always dial my default connection.  (If you are not online when you open a
Internet access program, such as a web browser or email client, you will get
the connection pop-up.)