Modem Dialup Problem

How do I fix this problem?
System is installed with windows 98.
From a cold boot the systems seems to load up normally.
I go to my connect icon and start my internet connection …. I hear the dial tone …. it goes for approx 15-30 seconds then the modem disconnects …. it will do this every time.
I close down the connect dialog box and go back to the desktop where I then double click on the My Computer Icon in windows and display my drives etc. and without doing anything else I close the My Computer windows and retry my dialup connection. It will then dial and connect evey time without fail.
I feel that this has to be some kind of windows setup problem caused by a registry key that isn’t setting itself properly on startup.
Thanks in advance