Its a problem with your modem on the Pentium 4. Try
looking for AT Initialization strings for it.

The abovementioned settings have nothing to do with your modem's
ability to maintain a connection.

I suggest you investigate your modem's last-call diagnostic report.
This will tell you the reason for disconnect, Tx/Rx error rates,
min/max Tx/Rx speeds, numbers of retrains and speedshifts, etc.

Assuming yours is a Conexant modem, then you can access this report by
using HyperTerminal to send the AT#UG or AT%V1 commands.

This site has general troubleshooting info:
http://www.modemsite.com/56k/trouble.asp

These links are more specific to your problem:
http://www.modemsite.com/56k/x2-inf.asp
http://www.modemsite.com/56k/x2-hyperterm.asp
http://www.modemsite.com/56k/usehyper.asp
http://www.modemsite.com/56k/diag.asp

If you find that your modem's error rates are high, then you can
improve the stability of the connection by limiting the connect speed
by adding the AT+MS command to your Extra Settings in DUN.

USR modems produce the most useful diagnostic report, including an
analysis of the frequency response of your phone line. This frequency
table will tell you a lot about what speeds are achievable under your
specific line conditions.

My own Rockwell chipped modem produces the following data. This is a
good session ...

TX/RX I-Frame count                 : 17654/21091
TX/RX I-Frame error count           : 29/14
TX Rate (Last/Init/Min/Max)         : 28800/26400/26400/28800
RX Rate (Last/Init/Min/Max)         : 46667/46667/45333/46667
Modulation/Protocol/Compression     : V.90/LAPM/V.42bis
Retrains (Issued/Granted/Fast)      : 0/0/5
Renegs (Issued/Granted)             : 2/0
Retrans per frame/Frames rejected   : 1/14
Error control timeouts in TX        : 16
Error control NAKs received         : 29
Termination Cause                   : Dte Hangup Command

... a not so good session ...

TX/RX I-Frame count                 : 10268/34101
TX/RX I-Frame error count           : 15/47
TX Rate (Last/Init/Min/Max)         : 28800/26400/26400/28800
RX Rate (Last/Init/Min/Max)         : 38667/42667/38667/42667
Modulation/Protocol/Compression     : V.90/LAPM/V.42bis
Retrains (Issued/Granted/Fast)      : 0/0/0
Renegs (Issued/Granted)             : 3/0
Retrans per frame/Frames rejected   : 1/47
Error control timeouts in TX        : 7
Error control NAKs received         : 15
Termination Cause                   : Dte Hangup Command

... and a bad session (water in the cable) ...

TX/RX I-Frame count                 : 784/4482
TX/RX I-Frame error count           : 23/211
TX Rate (Last/Init/Min/Max)         : 26400/26400/26400/28800
RX Rate (Last/Init/Min/Max)         : 38667/44000/33333/44000
Modulation/Protocol/Compression     : V.90/LAPM/V.42bis
Retrains (Issued/Granted/Fast)      : 0/1/2
Renegs (Issued/Granted)             : 12/0
Retrans per frame/Frames rejected   : 8/211
Error control timeouts in TX        : 16
Error control NAKs received         : 23
Termination Cause                   : Retrain Failed

Notice that the initial CONNECT speed is not a reliable indicator of
modem performance, as modems will speedshift as line conditions
change. Notice also that the modem can tell you the reason for
disconnect. For example, a "Termination Cause" of "Disconnect Frame
Received" indicates that your ISP kicked you off.

- Franc Zabkar