The error -50 is generic TCP/IP error that says the session died.

Check the server ACTLOG at the exact time and see what message was
issued on
the server end; you can usually at least tell whether the client killed
the
session or the server did.

Check the dsmsched.log file on the client, see if you get anything
consistent about WHERE the client dies, is it in the same part of the
filesystem each time, etc.