Mssql Locking causes Delphi App to crash

I have just gone live with a delphi app which has mssql at the backend.
Under load testing it has been well behaved. It is now installed in a
large health organisation with users coming in over wan connections to
the sql server. For periods the system performs well and then it
crashes. All other users of the system also crash out as well. The
application will still launch and they can even log on but meaningful
work is not possible.

I have reviewed the code and cannot find an issue there. It would be
easy for me to blame the network/ old pc's ( and not doubt the clients
IT support could point a the software) but I rather get to the bottom
of it. The sql server is not on a dedicated server. It is one of
these new fangled virtual servers and I wonder if this is the issue.
After all it is only using the chunk of memory allocated to it by the
real server it is on. Would this impact on performance. Any views
would be much appreciated.