Win XP modem +Legacy software[MOVED FROM WINDOWS]

Hi Guys
Was not Sure if this was the correct Forum, maybe Modem would have been better, Any way here goes...
I work for a EPoS Company our systems run on win xp, but we have third party software that runs in DOS mode, This software uses a USRobotics 56k Ext modem (Sporster). This is the only analoge modem we use.
Over the last few weeks these modems have started to not initialise on some of the systems. The Modem is allways available through windows.
which can also connect to the server using the Modem.
Software can not be at fault, we have been using this for years in this configuration. Like i said its only the past 2 months that this has started to happen.
What i should mention is that after swopping ports or restarting the modem they sometimes come back and initialise. If we get the said modem back to the office they also work. Anyone have a Idea what the cause is ??, we are out of ideas there appears to be no logic to this.
thanks in advance