Problem with Siemens ISDN Modem

I am using Windows XP and facing problems using Siemens ISDN Modem.

After 30 mins or an hour the connection stops working, messengers sign out and browsers show failure message. The connection doesn’t gets disconnected but it stops working.

If I disconnect it myself, then I need to restart windows because of failure modem message.

I also noticed that everytime I re-install the driver, it works fine. But after a reboot, it troubles me the same way.

Please help me.

Thanks.