DSC MD12 modem connection problems

[Tower Security <regsiemens@xxxxxxxxxxx>]

I have a DSC MD12 modem and it seems I often fight with getting a
working connection to panels that I know are programmed properly...in
fact if I go back to DLS1 and a 1200 baud modem on exactly the same
panel and I connect without issue.  This is all fine and good except
for those panels that can only be downloaded via DLS3 and the MD12
i.e. 1555mx.  I've tried forcing the baud rate in the DLS software to
various different settings and no joy.  Interestingly enough if I have
a tech on-site initiate a download it typically works fine.

Have I got a bad modem?  Any other suggestions?