Strange behaviour with Remote Access

Hi!
Pleeease help me with a very strange behaviour with remote access in Windows
XP Sp2 and Windows XP Embedded.
We have several pc:s that exactly behaves the same way. This has not been a
problem before for us.
Ok, here we go...
If you try to dial in to a system (a stand alone computer) it works fine.
The next time you try, it could work or fail. And here is the issue the times
it fails. Somehow the Routing and Remote Service has shut down (the DTR led
on the modem is off).
No one has touch the system, no power failure e.g.
Does anyone know if there is a workaround, fix or something you can do
(registry setting) to prevent this?
We have tried with different modems, different computers but with the same
result. So it looks like it is the ops Windows XP.
It is an external modem.
I appreciate any help or ideas I can get.

PS
If you power off the modem manually, why does not the Remote Access service
start up automatic (if the default setting is that the service is running
when you cycle the power)DS

Regards
Pat