Are you using KDE by any chance? If so, then it sounds like your connection to the dcopserver process (used to increase start-up times in KDE programs) is blocked somehow, or the process is being killed.
The first thing to do is to check if dcopserver is working when ppp is up, with the command
which should output one or more numbers if it's running.
Secondly, check your firewall configuration. Are you blocking on the lo (loopback) device? If so, check which ports are being blocked.
Hope that helps,
—Robert J. Lee