Try making resister S25 smaller. Default is 5. try S25=4  or

s25=3. When the software drops DTR, if the drop-length is smaller

than S25 then the modem ignores it. That software may be just  on

the edge and vary it (say between 4.9 and 5.1) so when it is  4.9

it would not drop the line. The chances are million(s) to 1  that

you'd get 5 modems in a row to fail the same way (unless ALL that

model  were  defective,  and that actually is  one  of  the  most

reliable models).  I use that model most, with no failure (and it

is on 24 hours a day for several months now), as do others  here.

I also wondered if the serial port could be the cause. Perhaps it

doesn't  keep DTR off in compliance with the software's  command.

It  may drop DTR and immediately restore it with no  'help'  from

the  software, or the voltage drop may not be within  specs  (ie;

does not drop 'low enough').