Simmens speedstream 4100 modem probs

Look at the log on your 4100. Select "keep log on restart".
Look for log outs and about 4 steps down Log ins (mine were about 20 seconds apart).
See if your modem reset (watch your lights) coincides with taking your phone off hook. Take phone off hook for about 20 seconds until you loose dial tone (mine would reset at this point about 50% of the time and show several log off-on in the log.
My phone company tech (SBC/AT&T) said that there engineering department had them install some line equipment at drop site that caused this problem. If you can do something to force the reset, then you'll get some action by your telco. Otherwise the testing they do will not show a problem.
If this does not work, contact your state PUC. Your phone company will be more cooperative when they are contacted by the PUC.